Depositional evolution of the Finnmark carbonate …...Depositional evolution of the Finnmark...

Depositional evolution of the Finnmark carbonate platform, Barents Sea: .results from wells 7128/6-1 and 7128/4-1

STEPHEN N. EHRENBERG, ERIK B. NIELSEN, TORE A. SV ÅNÅ & LARS STEMMERIK

Ehrenberg, S. N. , Nielsen, E. , Svånå, T. A. & Stemmerik, L.: Depositional evolution of the Finnmark carbonate platform, Barents

Sea: results from wells 7128/6-1 and 7128/4-1. Norsk Geologisk Tidsskrift, Vol. 78, pp. 185-224. Oslo 1998. ISSN 0029-196X.

Due to its extensive core coverage (444 m of the 527-m thick succession), well 7128/6-1 provides a unique opportunity for detailed

sedimentologic study of the entire sub-surface Late Carboniferous-Permian Finnmark carbonate platform. This 'reference section'

is augmented by 51 m of core in well 7128/4-1, 25 km to the west. Based on a combination of core description and a standard set of petrologic analyses performed at fixed l to 2 m spacing, the approximately 500-m thick carbonate succession is divided into

nine lithostratigraphic units, each comprising a distinct depositional facies association. Trends in microfacies, inferred water depth, cycle thickness, and gamma ray (GR) profile are used to differentiate a hierarchy of stratigraphic sequences, reflecting probable

5th- through 2nd-order fluctuations in relative sea leve!. Seven main sequences are recognized (3rd- or possibly lower-order}, whose

varying facies composition and cyclicity record the changing interplay of eustacy, climate, tectonics, and siliciclastic influx during the 50-60 m.y. history of platform evolution.

S. N. Ehrenbdg, Statoil, N-4035 Stavanger, Norway; T. A. Svånå, Statoil, postboks 40, N-9401 Harstad, Norway; E. B. Nielsen & L. Stemmerik, Geological Survey of Denmark and Green/and, Thoravej 8, DK-2400, Copenhagen N V, Denmark.

Introduction

Knowledge of the Upper Paleozoic succession of the Barents Sea is derived from seismic and well data and from outcrop sections at Svalbard and Bj<j>m<j>ya, along the northwestem margin of this roughly 1.5 million km area (Fig. l). On the eastem Finnmark Platform, these strata were tested by exploration wells 7228/9-1 (Norsk Hydro) in 1990; 7128/6-1 (Conoco) in 1991; 7128/4-1 (Statoil) in 1993; and 7229/11-1 (Shell) in 1994 (Fig. 2). Additional information was provided by a series of shallow wells drilled by IKU Petroleum Research near the erosional truncation of the platform succession south of the present study area (Fig. 2; Bugge et al. 1995). In well 7128/6-1, 444 m of core were recovered from the 901 m Upper Paleozoic succession, making this by far the most extensive1y cored Upper Pa1eozoic 1ocation in the Barents Sea. Because of the excellent core coverage and a nearly complete depositional succession, this well shou1d be regarded as the 'reference well' for the Upper Pa1eozoic strata of the Finnmark Platform. Well 7128/4-1, 25 km to the west, provides an important supplement to this reference section. lts 51 m of core in a key transitional part of the carbonate succession shows the nature of lateral facies and porosity variations over intermediate-scale distance.

Well 7128/6-1 was drilled to test two play models that continue to be of primary regional significance: (l) combined structural and stratigraphic trapping in Upper Permian seismic buildups, and (2) structural trapping in block-faulted Lower Carboniferous (Visean) sandstones. Well 7128/4-1 also tested these play models plus the

additional model (3) stratigraphic trapping in Upper Carboniferous/Lower Permian carbonates. Well 7128/6-1 was a dry hole, encountering oil shows in Upper Permian spiculite and at one horizon (2076 mCD) in Upper Carboniferous carbonates. Well 7128/4-1 made a technical oil and gas discovery in Upper Permian spiculite.

This paper is organized in two main sections: ( l ) description of the lithostratigraphy of the Upper Paleozoic, carbonate-dominated section in wells 7128/6-1 and 7128/4-1, with interpretation of the depositional settings represented and the style of cyclicity present; and (2) interpretation of the sequence stratigraphic evolution. Diagenesis and reservoir quality are covered in a second, companion paper (Ehrenberg et al. , this issue).

Regional setting

Following Caledonian (Late Silurian-Early Devonian) tectonism and metamorphism of the basement, EarlyMiddle Devonian extension produced thick siliciclastic sections in other parts of the Barents Sea and in surrounding areas (Steel & Worsley 1984; Johansen et al. 1993). No such strata have been penetrated on the Finnmark Platform, but locations of possible Devonian grabens have been outlined by gravimetric mapping. The known sedimentary section of the study area can be divided into four main intervals, as exemplified by the 2130 m-thick succession overlying the metasedimentary basement in well 7128/6-1:

• 339 m of Lower Carboniferous dominantly nonmarine siliciclastics;

186 S. N. Ehrenberg et al.

PALEOZOIC EXPLORATION TESTS: 1 7120/12-2 Norsk Hydro 1981 2 7120/12-4 Norsk Hydro 1984 3 7120/9-2 Norsk Hydro 1984 4 7120/2-1 Norsk Hydro 1985 s 7120/1-1 Shell 1986 s 7121/1-1 Esso 1986 1 7321/8-1 Norsk Hydro 1987 a 7124/3-1 Saga 1987 9 7226111-1 Statoil 1988 10 7228/9-1 Norsk Hydro 1990 11 7128/6-1 Conoco 1991 12 7229/11-1 Shell 1993 13 7128/4-1 Statoil 1994

STRUCTURAL FEATURES: HsB = Harstad Basin TB = Tromsø Basin SB = Sørvestnaget Basin

NORSK GEOLOGISK TIDSSKRIFT 78 (1998)

VP = Vestbakken Volcanic Province HB = Hammerfest Basin LH = Loppa High BB = Bjørnøya Basin SH = Stappen High GH = Gardarbank High NB = Nordkapp Basin TBB = Tiddly Banken Basin FH = Fedinskiy High CBH = Central Barents High SBH = Sentralbanken High OB = Olga Basin SBB = South Barents Basin NBB = North Barents Basin AH = Admirality High MZH = Mys Zhelaniya High NZB = North Novaya Zemlya Basin

Fig. l. Location of study area with respect to structural features and land areas in and around the Barents Sea.

NORSK GEOLOGISK TIDSSKRIFT 78 (1998) Finnmark carbonate platform, Barents Sea 187

Fig. 2. Locations of wells, seismic buildups, and

structural features on the Finnmark Platform and

adjacent areas. Salt intrusions in the Nordkapp

Basin are inferred to be derived from Upper

Carboniferous-Lower Permian strata deposited

during sea-leve! lowstands that exposed the

surrounding carbonate platforms.

Sakmarian-Artinskian buildups Late permian buildups

ø � �

Salt pillow Salt dome

o Land

• 527 m of Upper Carboniferous to Permian carbonatedominated rocks;

• 1135 m of Triassic siliciclastics; • 129 m of Quatemary glacial sediments.

The Upper Paleozoic strata form a monocline that dips approximately 2° northward at top Permian level (Fig. 3) as a result of both northward thickening of individual units and increasing Tertiary uplift toward the mainland. Three main tectonic episodes have affected these strata: ( l ) mid-Carboniferous (Late Visean-Kasimovian) rifting, forming half-grabens and tilted fault blocks, including the Nordkapp Basin and the structures on which the present wells are located; (2) Late Jurassic-Earl y Cretaceous uplift and faulting, which reactivated and enhanced the previously formed structures in the study area; and (3) Tertiary uplift, resulting in removal of around 1.0-1.5

50 km

km of section (Nyland et al. 1992). Mainly as a result of this uplift, progressively older strata are truncated by the Plio-Pleistocene unconformity towards the Norwegian coast (Fig. 3). The Upper Carboniferous to Permian carbonate-dominated succession thickens northward from roughly 0.5 km in the study area to something over l km near the Nordkapp Basin. Southward toward the erosional subcrop, individual platform units thin, apparently due to a combination of decreasing depositional thickness and erosion of both top and internal surfaces of each unit (Fig. 3). Thus the Norwegian mainland was a persistent positive element throughout Upper Carboniferous and Permian time, possibly undergoing episodic minor uplift events whose influence would be reflected in platform stratigraphy.

Distinction needs to be made between the Finnmark Platform as a structural element and the Finnmark car-


s 7128/12-U-01


7128/6-1 (projected) O N

Fig. 3. North-south seismic cross-section from 7128/6-1 7128/12-U-01 (location shown on Fig. 1). Vertical scale (two-way time) is not shown, but can be appreciated

from the depth scale given for well 7128/6-1 in Fig. 4. Colors indicate seismic reflectors shown in Fig. 4. Features to note include: ( l ) seismic anornaly just below base

Tatarian reflector in well location; (2) thinningftruncation of Upper Paleozoic strata toward the south; and (3 ) thickening from the structural crest into adjacent half-grabens of siliciclastics between Visean-11 and Middle Carboniferous reflectors.

bonate platform of Late Carboniferous/Permian age, although these en ti ties are large ly coincident in area. In Late Carboniferous through Permian time, the Barents Sea was part of a vast province of carbonate-dominated deposition that extended from the Canadian arctic to northern Russia and thence southward to the Caspian Sea (Faleide et al. 1984; Stemmerik & Worsley 1989, 1995; Heafford 1993). The Finnmark carbonate platform is a segment of this province that is bounded to the south by the erosional subcrop onto the Fennoscandian craton and to the north by a sharp increase in depositional paleo-slope, where platform carbonates pass laterally in to lowstand evaporites and deep-water facies of the Nordkapp Basin (Fig. 2). In the study area, the surviving platform dimension from south to north is roughly 80-150 km, but the original southern limit of the platform was an unknown distance south of the present subcrop.

The western and eastern limits of the Finnmark carbonate platform are poorly defined. To the west of 25°E latitude, the (structural) Finnmark Platform extends as a narrow stri p 20-60 km wide, where the depositional succession appears siliciclastic-dominated and, based on well 7120j12-4, may be entirely Upper Permian in age. To the southeast, the Carboniferous/Permian platform continues in a wide belt paralleling the Kola Peninsula (the Kola-Kanin Monocline) and thence into the Timan-Pechora Basin, where the platform succession has overall similarities to the Finnmark Platform (Johansen et al. 1993).

A general framework for understanding carbonate deposition in the study area is provided by paleoclimatic constraints. Paleolatitude increased from around 18°N in the mid-Carboniferous era to 38°N at the end of Permian

time, causing a change from earl y tropical to later temperate conditions (Steel & Worsley 1984). The interval from Late Carboniferous through Early Permian was characterized by global 'ice house' conditions, with major glaciations in the southern hemisphere and large high-frequency sea-leve! tluctuations (Veevers & Powell 1987). Under this regime, it is to be expected that platform areas would be repeatedly tlooded and exposed, and, given an arid climate, thick evaporites would accumulate in adjacent basins during sea-leve! lowstands (Wright 1992).

Previous work (Gerard & Burig 1990; Nilsen et al. 1993; Bruce & Toomey 1993) has shown that the Finnmark carbonate platform underwent four distinct stages of depositional evolution (Table 1), which are correlated to the well sections as shown at the left margin of Fig. 4. Broadly simi1ar depositional patterns are recognized throughout the Barents Sea, eastern Greenland, and north em Canada (Stemmerik & Worsley 1989; Beauchamp et al. 1989a). Replacement of photozoan biota by progressively cooler-water heterozoan biota (James 1997) from stages 2 through 4 retlects climatic cooling along the north em margin of Pangea, possibly in response to changes in oceanic circulation patterns in addition to gradual northward continental drift (Beauchamp 1994).

Analytical approach

Sampling strategy

Samples for the present study were taken at fixed one- to two-meter spacing throughout the cored intervals at locations already analyzed for porosity and permeability.

4

3

2

o

7128/6-1 Sequences

1900

1950

Vi sean-l!

Lt. Perm.

?Kung .

It. Artinsk.

e. Artinsk.

It. Sakmar.

J e. Sakmar.

J It. Asser.

J m. Asset.

] It. G'heL

J e. Gzhel.

J Kaslmov.

It. Moscov.

Brig ant.

Asb.

Holker.

IKU Wells

Lithostrat�raphic

� unit boun ary

Base-rever rise Sequence boundary Base-level fall MFS

CJ Dolomite

CJ Limestone

(ZJ ' Calcareous spiculile

� - Calcareous shate

(ZJ ' Spiculite

CJ Sandstone/siltstone

CJ Shale

c::=J:::l Buildups

Fig. 4. Lithostratigraphy, correlation, sequence stratigraphy, paleontologic dat ing, and porosity in wells 7128/4-1 and 7128/6-1 and the composite Upper Paleozoic section assembled by Bugge et al. ( 1995) from five !KU wells (labelled in center column). Taps of lithostratigraphic units (L-l through L-9 ) and major depositiona1 sequences (S-1 through S -7) are labelled. Stages of platfonn evolution (Tab1e 1) are 1abelled 'O' through '4' at far lett. Seismic reflectors (left) are given inforrna1.names used within Statoil. Cored interva1s are indicated by vertical bars. Gamma ray (GR) log scale is 0-150 API units for exploration wells and 0-300 APitmits for !KU wells. For !KU wells 7030/03- U-01 and 7129 /1 0-U-02 , wireline GR lags are not available, so core-surface GR lags (courtesy of Tom Bugge, !KU Petroleum Research) were nonnalized to match approximately the GR scale of the wire1ine lags. Displacement of the GR profile to the right in 7128/6-1 with respect to 7128/4 -1 reflects differences in mud composition (KCI-bearing mud in 7128/6-1) and centering of logging tool in drill-hole (tool pressed into contact with hole wall in 7128/6-1 ). In the exploration wells, porosity profiles are based on calibration of density log to core measurements. In the !KU wells, the porosity profile connects individual plug measurements. !KU seismic units of Bugge et al. ( 1995) are labelled in !KU GR co1umn. Short dashed lines labelled 'l' through '6' in the IKU porosity column are sequence boundaries of Stemmerik et al. ( 1995) Litho1ogy co1umn for well 7128/6-1 is based on core description (Appendix 1) and thin sections of sidewall cores and cuttings samples. Ages for well 7128/6-1 refer to fusulinid datings by V. l. Davydov, Inger Nilsson, and Groves & Walman ( 1997) and palynologic datings in Earl y Carbonifereous strata (0. McLean, University of Sheffield, unpublished 199 4 report for Norske Shell). Ages for !KU wells are from Bugge et al. ( 1995).

?Tat.-Kaz. ?Kung. -Ufim. Artinsl<.

Sakmar.

lt.Assel.

e. Assel.lt. Gzhel.

m.-lt.Gzhel.

e.Gzhel.lt. Kaslm.

l


The use of fixed-sample spacing is intended to give a statistically meaningful representation of the lithologies present, at the same time minimizing the possibility of observational bias when examining trends of sedimentologic and petrologic variation. The sample spacing applied is two meters from 1709-1809 mCD in well 7128/6- l , and one meter through all other cored intervals. This spacing is generally too wide to properly represent many thin beds and most surfaces, but much of this variation was examined using over 500 thin sections from locations between the fixed-interval samples. The results are presented in the core description log in Appendix l .

Microfacies

A petrographic thin section and polished core slab were prepared from all 396 fixed-interval samples analyzed from well 7128/6-1 and 52 samples from well 7128/4-1. For each sample, the Dunham classification was determined and the thin section was characterized using a standardized semiquantitative format. Abundances of 20 petrographic components were estimated by eye and assigned a score of O to 4 ( 4 = > 20% of the area of the thin section; 3 = 5-20%; 2 = 2-5%; l= <2%; O= none). These include 14 biological components (bryozoans, echinoderms, brachiopods, sponge spicules, foraminifera, fusulinids, corals, molluscs, ostracods, encrusting foraminifera/algae, Tubiphytes, non-phylloid algae, phylloid algae, Palaeoaplysina) and 6 non-biological components (siliciclastic sand/silt, clay, peloids, ooids, anhydrite, chert/macroquartz). Only the score for Palaeoaplysina was adjusted based on abundance estimated from the polished slab.

The above data were then used to assign each sample to one of 12 microfacies (Table 2), which are defined

Table l. Stages of Finnmark Platform evolution.

O. Pre- & syn-rift siliciclastics (Visean-Bashkirian)

• Overall transgressive trend from fluvial to delta-plain to nearshore marine

l. Mixed siliciclastic-carbonate

(Moscovian-Gzhelian =L-l and L-2) • Waning tectonism • Alternating clastic-carbonate sedimentation

2. Shallow-water photozoan limestone + dolomite (Gzhelian-Sakmarian = L-3 through L-7)

• Warm-climate 'chloroform' (algal-formaminifera) biota e Meter-scale shoaling-upwards cycles • Low-relief Pa/aeoap/ysina -algal buildups e Evaporite deposition & dolomitization • 2-4 km halite & gypsum in Nordkapp Basin (probably during sea-leve!

lowstands corresponding with platform exposure)

3. Open-marine heterozoan limestone

(Sakmarian-Artinskian = L-8)

• Temperate-climate 'bryonoderm-extended' (bryozoan-echinoderm) biota

e No meter-scale shoaling-upwards cycles

e Large microbial-mud/bryozoan buildups along shelf edge

4. Deepfcold-water calcareous shale, limestone, & spiculite

(mid-Late Permian = L-9) e Cold-water 'hyalosponge-bryonoderm' biota

• Two thick cycles with shaly bases shoaling upwards into spiculite

• T erminated by regional clastic influx

Finnmark carbonate platform, Barents Sea 189

broadly enough to include all of the uniformly-spaced samples examined. The thin-section data were then interpolated in conjunction with macroscopic core descriptions to generate a continuous sedimentology/microfacies log of the core (Appendix 1). Finally, the 7128/6-1 core log was checked against some 436 thin sections prepared independently by Conoco, most of which are located at depths in between the fixed-interval Statoil samples, along with 76 infill thin sections prepared for the present study.

The term 'buildup' (microfacies MF-6) is used to indicate rocks which, in most cases, have a fabric dominated by the platey organisms Palaeoaplysina and phylloid algae. These rocks have widely varying amounts of muddy, typically micro-peloidal sediment between the plates, thus corresponding to the complete spectrum from wackestone to grainstone. Typically the fabric of the Palaeoaplysina buildups appears to be a fragmenta! accumulation of plates, but some cases may be true boundstone (baffiestone or bindstone, depending on the interpreted life-orientation of Palaeoaplysina ). Systematic diffe�entiation between various gradations of plate-dominated fabrics corresponding to boundstone, wackestone, etc. is often ambiguous due to diagenesis; such differentiation has not been attempted in the present study. Instead, all MF-6 intervals have been plotted in the 'boundstone column' of the sedimentology log in Appendix l , with the understanding that a spectrum of fabrics is in fact present. These rocks may be considered varieties of 'segment reef (Braga et al. 1996), in that they consist of variably-fragmented 'segments' of mound-dwelling organisms lithified by microbially-generated peloidal mud matrix (Pickard 1996).

Field studies on Spitsbergen, Bjørnøya, and more distant locations show that the lithologies included in MF-6 typically have morphologies of low-relief lenses to banks from < l to as much as l O m thick, and from a few m to a few km in lateral extent, but commonly stacked or amalgamated into thicker, more extensive buildup complexes (Skaug et al. 1982; Stemmerik et al. 1994; Johansen & Eilertsen 1995). It has been demonstrated that these deposits commonly possessed topographic relief on the seafloor, thus fulfilling Wilson's (1975) definition of the term 'buildup'.

Phylloid algae is main1y of the codiacean variety in the lower part of the succession, but in units L-6 and L-7 the phylloid plates are mainly red algae resembling Archaeolithophyllum. Other algaes present include Dasycladaceans, Beresellids, and minor occurrences of Stachaeoides, and Fourstonella.

Also present in the cores are subordinate amounts of several other types of boundstone, where the binding/baffling material is either coral or various encrusting organisms (tubular foraminifera, algae(?), Tubiphytes, serpulids, bryozoans). For convenience, these varieties have also been included in MF-6, with the dominant type of binding organisms indicated by symbols on the sedimentology log in Appendix l .

The carbonate grains present in the studied intervals are, in nearly all samples, dominantly to exclusively bioclasts.

1 90 S. N. Ehrenberg et al. NORSK GEOLOGISK TIDSSKRIFT 78 (1998)

Table 2. Microfacies (MF) descriptions. The term 'shallow' refers to depths within which biota are dominantly phototrophic. Thus ' deep' refers generally to depths

greater than 50- 1()0 m, depending on water clarity. @ indicates microfacies commonly containing anhydritejsilica nodules.

MF·!

Ex tent

Lithology

Bi o ta Environment

MF-2

Ex tent Lithology

Biota

Environment

MF-3

Ex tent

Lithology

Biota

Environment

MF-4

Ex tent

Lithology

Bi o ta

Environment

MF-5

Extent

Lithology

Biota

Environment

MF-6

Ex tent

Lithology

Biota

Environment

MF-7

Ex tent Lithology

Biota

Environment

MF-8

Extent Lithology

Bi o ta

Environment

MF-9

Extent Lithology Biota

Environment

MF-10

Extent

Lithology

Bi o ta

Environment

MF-Il

Extent

Lithology

Biota

Environment

MF-12

Ex tent

Lithology

Biota

Environment

Calcareous ftne-grained splculite and silty shalc

Lower cycie of L-9. Two sub-vareties are distinguished. These commonly occur thinly interlayered, or with MF- lA as lenses or nodules within MF- lB . Shales may have subordinate spicule

content, and spiculites may contain minor silt. Both are bioturbated, with varying assemblages of burrow types.

MF- l A: Calcareous fine-grained spiculite. Chert-filled moids of spicules (20-50 microns diameter) in turbid carbonate matrix, commonly doiomitized. Light grey with

nodular fabric. MF- l B: Calcareous silty shale, commonly spiculitic, typically containing 15-40% very fine (20-100 micron) bioclastic calcite particles. Dark gcey with laminated fabric.

Siliceous monaxon sponges, brachiopods, ostracods, echinoderms, foraminifera, ammonites.

Deep water, well below storm wave base.

Coarse spiculite

Up per cycle of L-9. White to light grey spiculite with subordiate lenses of light grey to green sitty shale. Large-diameter monaxon spicules (100-500 microns) in matrix of small spicules

(20-50 microns) with variable abundance of interstitial c1ay and silt; commonly burrowed.

Siliceous sponges with subordinate brachiopods, echinoderms, minor small foraminifera. Moderately deep water, up to storm wave-base.

Byozoan-echinoderm wackestone

Mainly L-2 and L-8; minor occurrences in L· l , L-3, L-4, L-6, L-7, and L-9.

Wackestone, commonly partly chertified, with widely varyng proportions of silty, shaiy matrix. Frequent bioturbation and layers rich in silty shale.

Similar to MF-4, but generally without Tubiphytes and with frequent siiiceous sponge spicules.

Open marine, near or below storm wave base.

Bryozoan-echinoderm grainstooefpøckstone

Mainly L-8; minor occurrences in L- l , L-2, L-3, L-4, L-6, and L-7.

Bioclastic grainstone to packstone. Typically homogeneous fabric may refiect ubiquitous bioturbation.

Bryozoans, echinoderms, and subordinate brachiopods, Tubiphytes, and fusulinids; minor small foraminifera and red algae.

Open marine, between fair-weather and storm wave base.

Fusulinid/foraminifera wackestonefpackstone @

Mainiy L-3, L-4, L-5, and L-7; only one occurrence each in L-l and L-6.

Bioclastic wackestone to packstone. With decreasing mud con tent, this microfacies is gradational to MF-7. Similar to MF-7, but typically witb predominance of fusulinids and generally lacking algae. A variant consisting of echinidenn wackestone with minor or no fusulinids is

also included in this microfacies for convenience. In some cases, the biota is unidentifiable due to leaching and dolomitization, and the microfacies can only be inferred.

Shallow water, below fair-weather wave-base. The biota commonly has limited diversity, most likely indicating increased satinity. Elsewhere, MF-5 has higher diversity

and contains bryozoa, indicating normal marine conditions.

Buildups @

L-i through L-7. The fabric is in most cases dominated by plates of Palaeoaplysina or phylloid algae, which are commonly encrusted and fragmented. Micro-peloidal Biota matrix or

cement with variable bioclast content typically fills in between plates, with wide variations in the ratio of plates to matrix in different buildups. For conveneience,

this microfacies has also heen used to inciude other, relatively subordinate types of bafflestonefboundstone, containing either coral or encrusting forms (including

varying combinations of tubular foraminifera, ?atgae, Tubiphytes, serpulids, bryozoans). The dominant type of binding/baffl.ing organisms present in each buildup are

indicated by symbols on the sedimentoiogic log in Appendix i .

Palaeoaplysina and, or phylloid algae. Other components present in varying combinations are foraminifers, fusulinids, aJgae, corals, bryozoans, brachiopod.s, and echinoderms.

Shaltow water, below fair-weather wave base.

Foraminifera grainstonefpackstone @

Mainly L-6 and L-7; lesser occurrences in L-l through L-5.

Biociastic grainstone to packstone, localiy rich in quartz sand and thus gradationai to MF-l i. A local variant occurring at only one piace in the 7128/6-1 core is oolite

grainstone.

Abundant foraminifers (commonly encrusting) with subordinate fusutinids and algae. Echinoderms, corals, Tubiphytes, encrusting worms, bryozoa, brachiopods,

ostracods, gastropods, and phylloid algae occur in varying combinations.

High-energy, shallow-water shoals. The biota commonly has limited diversity, most likely indicating increased salinity. Elsewhere, MF-7 has high diversity and contains bryozoa. indicating normal marine conditions.

Dolomitic mudstone @

Mainly L-3 and L-5; lesser occurrences in L-l and L-7. Dolomitic mudstone and subordinate wackestone with anhydritejsilica (chalcedony and macro-quartz) nodules and variable amounts of siliciclastic sand and silt. Some

intervals appear barren, but other intervals contain bioclasts. Frequent stratal discontinuities in some intervals may record episodes of subaerial exposure. Very limited diversity, including siliceous sponge spicules, brachiopods. fusulinids, solitary corals, or (most commonly) no recognizable fossils. Same intervaJs have no

fabric apart from anhydrite/silica nodules. while other intervals show bioturbation. Some intervals have algal(?) lamination. Hypersaline shelf/lagoon to intertidal to sabkha.

Shale

Mainly L-2 and the lower cycle of L-9; lesser occurrences in L-1 and L-7. Carbonate content and siliciclastic Biota silt both vary widely from abundant to absent.

Variable from barren (with or without Environment bioturbation) to similar to MF-l and MF-3 .

Lower shorefacejoffshore.

Calcsreous siltstone

L·2.

Silicic1astic silt to very fine-grained sand makes up 60% or more of the grain fraction. Subordinate amounts of larger calcite bioclasts are commonly present. Grains are

tightly cernented by fine calcite spar and syntaxiat spar. Same cases are gradational to MF-3. Similar to MF-3. Lower shoreface.

Sandstone @

Mainly L-l and L-2; lesser occurrences in L-3, L-4, and L-7.

SilicicJastic sand or silt makes up 40% or more of the grain fraction. Carbonate content is widely variable, but conunonly high. Same samples are gradational to MF-7.

Where present, biota is simiiar to that of MF-7 or MF-3. Nearshore marine.

Anhydrite @

L-5 and L-6 (one fixed-intervai sample in each).

Nodular anhydrite with fine-felty fabric.

None.

Evaporitic lagoon.


Thus the adjectives 'bioclastic' or 'skeletal' should be taken for granted in connection with the terms grainstone, wackestone, etc. throughout this text. Exceptions to this include various sandstone beds, where carbonate grains may be dominantly peloids and lithoclasts. There is also a single occurrence of ooid grainstone at 1 93 1 mCD in well 71 28/6- 1 .

Inferences about restriction and exposure

Interpretation of restricted, hypersaline depositional conditions in certain intervals is based on the characteristic association of limited fauna! diversity with early dolomitization and nodular anhydrite. We also place particular emphasis on the presence or absence of bryozoa, as this group is believed to have relatively low tolerance for anything other than normal marine salinity, and is particularly ubiquitous in the open-marine intervals of the studied strata. However, the presumed salinity significance of Paleozoic bryozoans is little documented in the literature (see, for example, Smith 1995),

and is largely based on the general observation of bryozoan abundance correlating with intervals having open-marine characteristics (Nakrem 1994). Fig. 5 compares the distribution of bryozoan abundance in eight of the microfacies defined in Table 2. This supports the general picture of maximal restriction in MF-8; variable but commonly significant restriction in MF-7, MF-5, and MF-6; open-marine circulation in MF-3 and MF-4; and variation due partly to restriction, but more importantly to clastic influx, in MF-3 and MF-11.

100

o 100

-c:

� Q)

a..

MF-3 (N=41 )

MF-7 (N=82)

MF-4 (N=51 )

M F-8 (N=46)

0 1 2 3 4

M F-5 (N=36)

M F-1 0 (N=1 9)

Bryozoan abundance

M F-6 (N=79)

M F-1 1 (N=28)

0 1 2 3 4

Fig. 5. H istograms showing percentage frequency distributions of bryozoan

abundance scores (4 = > 20"/o of the area of the thin section; 3 = 5-20"/o; 2 = 2-

5%; l = < 2%; O = none) in each of 8 microfacies (Ta ble 2), including all samples

from wells 7 128/6-1 and 7 1 28/4- 1 . N = number of samples represented. Bars in

each histogram sum to l 00%.

Finnmark carbonate platform, Barents Sea 1 9 1

Exposure surfaces are likely to be present at numerous horizons in units L-1 through L-7 because these shallow-water carbonates were deposited during a time of large-amplitude, high-frequency sea-level fluctuations (Veevers & Powell 1987). Outcrop studies of similar age deposits from Spitsbergen and Bjørnøya (Pickard et al. 1 996; Stemmerik & Larssen 1 993) demonstrate that exposure surfaces typically have great lateral variation in the degree of development of various exposure criteria. Consequently, the recognition of such surfaces in onedimensional core studies is expected to be highly problematic. Microcodium is a common indicator of exposure in the strata on Spitsbergen (Skaug et al. 1 982), but has been found in only one place in the cores of the present study: as tubular (apparently in situ) structures in thin limestone lenses having stylolitic contacts within a 2 cm-thick shale layer at 1 886.95 mCD in well 7 128/6- 1 . Exposure surfaces are inferred at specific locations in the cores, in general based on the occurrence of a sharp discontinuity where a deeper-water facies overlies relatively shallow-water deposits, combined with various additional criteria. These include: ( l ) truncation of laminations or plates at sharp, irregular contacts; (2) infilling of apparent dissolution cavities in the underlying layer by the overlying sediment; (3) possible rip-up clasts of the underlying layer included in the immediately overlying sediment; and (4) in MF-8 strata, a layer of small anhydrite nodules overlain by shale Jaminations at a sharp contact. Nowhere is the interpretation of exposure unambiguous, and most of these surfaces could alternatively be explained as the products of submarine erosion or cementation (Shinn 1 986). The less ambiguous locations are indicated in the core description log in Appendix l .

Quantitative compositional indexes

Bulk XRD and bulk chemical analyses were done at one-meter spacing throughout 232 m of the 7 1 28/6- 1

core (2045- 1 835 mCD and 2 1 2 1 -2101 mCD), and additional bulk chemical data have been acquired to fil! in gaps in this initial coverage. Bulk XRD analyses were performed by Paul H. Nadeau at Statoil. Bulk chemical analyses were done by XRAL Laboratories, Toronto, Canada. The bulk XRD and bulk chemical analyses were used to derive two indexes of bulk-rock mineralogy, plotted in Appendix l . The bulk XRD analysis of each sample was used to calculate the relative proportions of weight percent calcite and dolomite, expressed as:

CCD = 1 00 x Calcite/(Calcite + Dolomite).

The bulk chemical analysis was used to calculate the weight % of total siliciclastic material plus biogenic/authigenic silica (Siliciclastic + Chert Index = 'SCI'):

SCI = wt. %Si02 + Al203 + Ti02 + Fe203 + N� O + K20

+ P205 + H20*


Table 3. Depositional setting, lithology, age, and thickness of lithostratigraphic

units forming the Upper Paleozoic succession in wells 7128/6-1 and 7128/4-1.

Thickness (m)

Unit Depositional setting, lithology and age 7128/4-1 7128/6-1

L-9 Deep-water spiculite, limestone, and spiculitic to 135 123

silty shale

(?Kungurian-Late Perrnian)

L-8 Open-shelf limestone 104 89 (late Sakmarian-late Artinskian)

L-7 Offshore to lower-shoreface shale and 27 33

shallow-water limestone

(early Sakmarian)

L-6 Shallow-shelf limestone 36 41

(middle Asselian-early Sakmarian)

L-5 Lagoon/sabkha dolomitic mudstone and 37 30

shallow-water packstone

(middle Asselian)

L-4 Shallow-water wackestone and buildups (partly 71 81

dolomitized)

(late Gzhelian -earl y Asselian)

L-3 Lagoon/sabkha dolomitic mudstone 21 29

(middle-late Gzhelian)

L-2 Offshore to lower-shoreface shale, siltstone, and 40 52

silty limestone

(Kasimovian-early Gzhelian)

L-l Shallow-water sandstone (partly dolomitized) and 18 51

limestone

(late Moscovian)

upper Visean nearshore marine shale/sandstone unit 71 55

(Asbian-Brigantian)

Visean delta plain coaly/shaly unit 222 153

(Holkerian-Asbian)

Visean tluvial sandstone unit 153 131

(middle Visean: ?Holkerian)

where H20* = weight loss on heating to 850°C(C02 + S03). This index is based on the observation that the subject rocks consist of four genetic components:

l. Car bona te ( calcite and dolomite ); 2. Evaporite (anhydrite and less commonly gypsum); 3. Siliciclastic (quartz, feldspars, and clays); and 4. Biogenic plus authigenetically introduced silica (chert,

chalcedony, macroquartz).

There are also minor amounts of authigenic barite and fiuorite in several samples. The SCI sum should be equivalent to the weight percentage of components (3) + (4), as the SCI elements are present only in trace amounts in the minerals composing the first two components. Inclusion of Fe203 and P205 in SCI is supported by the overall correlation found between these elements and alumina. On the core data profiles (Appendix 1), SCI may be compared with the thin-section score of siliciclastic sand + silt content (ranging from O to 4, as explained a bo ve).

Total sulfur content was used to calculate the weight % anhydrite or pyrite in each sample, using the knowledge from the bulk XRD analyses that these are the only two sulfur-rich minerals present in this sample set and that they tend not to occur together in the same sample. Anhydrite contents calculated from the bulk chemical analyses are believed to be fairly accurate (the main error involves the assumption of stoichiometry)


and are typically much lower than the amounts determined by bulk XRD.

Core-to-log shifts

Careful attention has been given to accurate correction between core depth (CD) and log depth (LD) along each segment of the cored intervals. These corrections are substantial in many places, and one 16 m-interval of the core (2005.25-2021.60 mCD) was even found to have been mistakenly turned upside-down in the course of handling. Depth shifts were determined mainly based on comparison between the gamma ray (GR) logs run in the well and along the surface of the core before slabbing. However, additional adjustments of up to a meter were found to be necessary for several intervals when a second GR profile was run along the slabbed surface of the viewing cut of the core. In the present report, the wireline GR log has been shifted to match core depth (Appendix l) because this is the primary reference scale for both the core description and the petrologic sampling.

Lithostratigraphic description and depositional interpretation

Resting on probable Caledonian metasedimentary basement, the earliest strata encountered in the well locations are mid-Visean fiuvial sandstones, which mainly predate mid-Carboniferous block faulting. Overlying, finer-grained fioodplain sediments of Late Visean age thin onto structural crests, refiecting syn-rift deposition, and show increasing marine infiuence upward. These Lower Carboniferous siliciclastics are separated from the overlying Upper Carboniferous carbonate platform by an erosional unconformity and major hiatus that is recognized throughout the Barents Sea, Svalbard, and northern Greenland (Stemmerik 1997).

The Moscovian-Upper Permian biogenic platform interval is divided into nine lithostratigraphic units, L-1 to L-9, based on biota and facies associations (Table 3; Fig. 4). Fig. 6 provides an overview of the lithologic make-up of each unit in terms of the 12 microfacies defined in Table 2. These histograms should provide an accurate representation of the overall composition of each interval, because they are normalized to relative percent in each unit and are based on uniform sample spacing through each unit. Unless otherwise stated, the following descriptions refer specifically to well 7128/6-1. Fig. 7 shows estimated regional correlations for the well sections within three frames of stratigraphic reference: (l) the composite Upper Paleozoic section cored in the shallow IKU wells just south of the study area (correlation also shown in Fig. 4); (2) the four Finnmark Platform seismic units of Nilsen et al. (1993); and (3) exposed strata on Spitsbergen and Bjørnøya (Fig. 1).


Pre-carbonate strata

Basement. - The sedimentary section rests on a basement of very low-grade metasedimentary rocks of uncertain depositional age (probably Late Precambrian to Early Paleozoic). These strata were probably deformed and eroded during the Caledonian orogeny (roughly 420-390 Ma). Several meters of core were recovered from the basement in each of the studied wells, consisting of cross-bedded, medium- to coarse-grained arkoses (braided stream deposits) in well 7128/6-1, and nearly

100

-c

� Q)

a..

Unit 1 (N=32)

o�� 100

-c Q) e Q)

a..

Units 3 & 5 (N=58)

o��Lr

100

-c

� Q)

a..

Unit6 (N=72)

OLr��Lr�� 100

-c

� Q)

a..

Unit8 (N=57)

Microfacies

Unit2 (N=53)

Unit4 (N=81)

Unit7 (N=52)

Unit 9 (N=43)

9101112

Microfacies Fig. 6. Histograms showing percentage frequency distributions of microfacies

(Table 2) in each lithostratigraphic unit, including samples from wells 7128/6-1

and 7128/4-1. N = number of samples represented. Bars in each histogram sum to

100%.


vertical beds of black shale and subordinate fine-grained micaceous sandstone in well 7128/4-1. K-Ar analysis of illite ( < 0.2 micron fraction) from sandstones in the basement cores gave dates of 340 Ma in well 7128/4-1 (P. H. Nadeau, Statoil, pers. comm.) and 135 Ma in well 7128/6-1 (J.-P. Stiberg & G. Åberg, Institute for Energy Technology, unpubl. report, 1993).

Visean sandstone unit. - The top of this unit corresponds with the 'Visean-1' seismic reflector (Figs. 3 and 4). Seismic mapping indicates relatively uniform thickness of 100-200 m in the study area, suggesting deposition prior to or in the earliest stages of Carboniferous rifting. This unit consists of alluvial fan and braided stream channel deposits, overlain by floodplain facies. A 27.5-m core in the upper, floodplain-facies part of this unit in well 7128/4-1 (Fig. 4) consists of fine- to medium-grained sandstones with minor clay-rich deposits and coal.

Visean coaly jshaly unit. - The top of this unit corresponds with the 'Visean-II' seismic reflector (Figs. 3 and 4). Thinning and erosional truncation of this unit on structural highs, combined with thickening in the half grabens, reflects the onset of Late Visean to ?Kasimovian rifting. However, thickness variations between structural highs and lows are still smaller than in the overlying unit. A 27.4-m core was recovered from the upper part of this unit in well 7128/6-1 (Fig. 4). The lithology consists of alternating claystone, siltstone, finegrained sandstone, and coal beds, interpreted to be deposited in a delta-plain or coastal floodplain environment. About 25 km north of the wells studied, this unit develops a strong northward progradational pattern on seismic lines, indicating northward transition into a prograding coastline.

Up per Visean shale fsandstone unit. - The top of this unit corresponds with the 'middle Carboniferous' seismic reflector (Figs. 3 and 4). These strata were deposited during the most active rifting phase, with thick (up to 450 m) clastic wedges filling in the graben areas. Northeastsouthwest trending tilted fault blocks, and halfgrabens which formed during this rifting event are the dominant structural features of the eastern Finnmark Platform. The studied wells are located in crestal positions on two such fault blocks, where relatively thin successions were deposited during this time interval (Fig. 3). In the well locations, the lithology consists mainly of shale and sandstone, interpreted to be dominantly marginal marine deposits. The youngest coal beds occur near the base of this unit, but palynofacies analysis indicates dominantly marine depositional conditions throughout the unit. In well 7128/6-1, a 5-m limestone bed occurs at the base of the unit. Thin sections of cuttings show this to consist mainly of peloid packstone to grainstone with abundant brachiopods and foraminifera.


L-1 (Late Moscovian)

The base of unit L-1 (2150 mLD; roughly 18 m below the base of the cored interval in well 7128/6-1) coincides with the 'middle Carboniferous' seismic reflector, which clearly shows truncation of underlying strata in many areas. Based on paleontologic datings of Late Moscovian in the central part of L-1, and Brigantian throughout the underlying upper Visean shalejsandstone unit, this surface may represent a time gap of between 22 and 33 million years (Harland et al. 1990). The basal surface is overlain by 13 m of conglomeratic sandstone, mainly containing pebbles of sedimentary and metamorphic quartzite. A small proportion of sandstone fragments from cuttings in this interval contains abundant bitumen.

Unit L-1 marks the waning of tectonic activity and the initial phase of the establishment of a stable carbonate platform. Seismic data show that the correlative interval thins over structural highs, as in the present well locations, and thickens into surrounding graben areas. However, thickness variation is much less than in the underlying upper Visean shalejsandstone unit. In well 7128/6-1, this interval consists of alternating marine sandstone and carbonate, but the lithology is thought to become carbonate-dominated with increasing distance from sources of siliciclastic influx. This is supported by data from well 7226/11-1 on the northern margin of the Nordkapp Basin

Ma Age

Tat.

Kaz.

260 Kung.-UI.

c: as Art.

'§ Q) a..

280 Sak .

Assel.

Gzhel.

300 Kaslm.

Mose.

Ul :::1

Bas hk. e Q) 320

:t:: c: o -e SØrp. as (.)

340 Vise.

Tour. 360


(Fig. 2), where the correlative unit consists of roughly 250 m of shallow-water limestone directly overlying Caledonian-age amphibolite facies schist.

Regional correlation of unit L-1 is summarized in Fig. 7. Correlation to the IKU cores is uncertain, where a possible equivalent is an 87 m-thick interval of sandstone with subordinate siltstone and shale (the middle Carboniferous seismic unit of Bugge et al. 1995, interpreted as fan delta to marine deltaic deposits). L-1 is correlated with the regional seismic unit I of Nilsen et al. (1993), the exposed equivalents of which are suggested to be the Kapp Kåre and Kapp Hanna Formations of Bjørnøya, and the lower Nordenskioldbreen Formation (Minkinfjellet Member) of Spitsbergen (proposed changed to Minkinfjellet Formation; Dallmann et al., in preparation).

Facies association. - The cored portion (upper 64%) of L-1 can be divided into three limestone intervals and three siliciclastic-rich dolomitic intervals. The limestones include thin phylloid algae buildup beds (MF-6), foraminifera- to bryozoan-dominated grainstone to wackestone (MF-7, MF-5, MF-3), and calcite-cemented sandstone rich in bioclasts and lithoclasts (MF -11 ). The limestone intervals represent open-marine, shallow-water conditions with periodic coarse siliciclastic influx.

Billefjorden Gp. Nordkapp Fm.

Røedvlka Fm.

Fig. 7. Ages and suggested regional correlation of Upper Paleozoic lithostratigraphic units in well 7128/6-1. Vertical axis is absolute time scale of Harland et al.

(1990). Triangles under 7128/6-1 depict major depositional sequences. Diagonal ruling indicates non-deposition. On the Finnmark Platforrn, however, evidence is

inconclusive as to whether the upperrnost Perrnian should be represented as a hiatus or by continuous deposition. Colors indicate approximate correlative units of

similar lithology.


The dolomitic intervals include two distinctive lithologies: (l) dolomitic sandstone ( MF-11) and (2) interlayered d?�omitic mudstone (MF-8) and shale ( MF-9). The dolomttlc sandstone beds are 3-6 m thick and show trends of �pward-decreasing GR activity, although no correspondmg trends in grain size or clay and carbonate content have been detected in the thin sections. Low-angle cross-bedding or lamination with common bioturbati?n is present throughout. Sorting is typically distinctly b1modal, with laminations of coarse, very well-rounded quartz and K-feldspar grains variably intermixed with very fine- to fine-grained sand. The lowermost sandstone interval is cemented by dolomite spar and contains abundant bioclasts (mainly bryozoa, echinoderms, and unidentified micritic grains). In the upper two sandstones, the sand grains are coated by microdolomite cement, such that few grains are in contact. These sandstones contain no calcite bioclasts, but abundant empty or anhydrite-filled grain molds may represent former bioclasts. The bulk chemical data show that the sandstones consist of 40-60% carbonate minerals, and thus should be thought of as comprising 'siliciclastic-rich' rather than 'siliciclastic-dominated' intervals. The sandstones are interpreted as progradational marine shoreline deposits based on their fabric and upward-cleaning GR profiles. However, pro bable aeolian transport of the sand before marine deposition is suggested by the bimodal sorting, the striking roundness of the coarser fraction and the low clay content. '

The dolomitic mudstone and shale occur as three thin (l-2 m) intervals that directly overlie or underlie the dolomitic sandstones. The mudstone in two of these intervals (2111 and 2121 mCD) contains bioclasts and gl�uconi�e pellets. Shales in these intervals correspond w1th maJor GR peaks. A fourth thin interval overlying the lowermost dolomitic sandstone (at approximately 2127 mCD) should probably also be included in the same �ateg�ry. It co�sists of thin (l-3 cm) shales correspondmg w1th a maJOr GR peak and interlayered with partly dolomitized, bioclast-rich siltstonejsilty wackestone (MF-3).

Cyclicity. - The cored portion of unit L-1 is interpreted as comprising three main cycles, each consisting of the following three zones: (l) a thin basal zone of high-GR shale and dolomitic mudstone; (2) a thick dolomitic sandstone; (3) thinly bedded calcareous sandstone and limestone. The basal part of the lower cycle is inferred to be 2-3 meters below the base of the cored interval. This zonal sequence is interpreted in terms of a single (4thorder?) transgressive-regressive depositional cycle containing smaller sub-cycles. The basal shalejmudstone and overlying dolomitic sandstone record transgression and marine reworking of aeolian sand, involving alternating periods of hypersaline and normal marine waters. Continued sea-level rise resulted in highstand deposition of thin apen-marine limestonejcalcareous sandstone subcycles unaffected by hypersalinity.


L-2 (Kasimovian-early Gzhelian)

Unit L-2 consists of 7 upward-coarsening shale-silts�one-wackestone cycles. The lower boundary is identlfied based on a major change in the lithology and facies of the siliciclastic intervals: from sand-rich, nearshore marine in L-1, to clayjsilt-rich, lower shoreface in L-2. This boundary coincides with the 'Upper Carboniferous' seismic reflector, which shows indications of truncation of underlying strata in same areas, for example near well 7128/4-l , but appears conformable on most seismic lines.

Facies association. - L-2 is made up mainly of three microfacies. Shale to sil ty shale (MF -9) was deposited in a deep off shore environment and represents the maximum water depth and minimum accumulation rate in each cycle. Calcareous siltstonejvery fine-grained sandstone (MF-10) represents a lower shoreface environment based on the open-marine fauna, abundant bioturbation' grain size, and generally high clay content. Silty wacke� stone (MF-3) represents a similar environment, but with shallower water or lesser siliciclastic influx, allowing greater carbonate production. MF-10 and MF-3 rocks in �-2 show all gradations toward intermediate compositlons, and contain similar fauna, consisting of bryozoans, echinoderms, and brachiopods, with variable presence of fusulinids, corals, gastropods, and trilobites. Siliciclastic grain size mostly borders between silt and very fine sand. Both siltstones and wackestones are heavily bioturbated throughout, including wide diversity of trace fossils.

Cyclicity. - Cycle bases consist of shale with little or no carbonate content, passing upward into silty and calcareous shale and thence into calcareous siltstone and silty wackestone. In general there is an upward increase in the proportion of carbonate towards the top of most cycles, reflecting decreasing water depth. The lower cycle in unit L-2 is 25 m thick, and comprises two sub-cycles, each capped by a phylloid algae-Palaeoaplysina buildup. The top l O cm of the upper buildup is brecciated and overlain by 40 cm of packstonejwackestone with rip-up clasts, suggestive of subaerial exposure followed by transgress�ve reworking. This basal 'megacycle' is overlain by a senes of 5 progressively thinner cycles. They resemble the shale to wackestonejsiltstone portion of the basal cycle, but lack shallow-water capping facies comparable to the buildups of the basal cycle. According to the terminology of Soreghan & Dickinson (1994), the basal cycle is a 'catch-up' cycle, while the overlying cycles are 'give-up' cycles that almost managed to catch up before being overwhelmed by the succeeding sea-level rise.

L-3 (middle-late Gzhelian)

Unit L-3 represents an important transition from siliciclastic-rich, deeper-water sedimentation to carbonatedominated, shallow-water conditions. Silty dolomitic mudstone (MF-8) is the dominant lithology, and the


lower boundary of L-3 is picked at the base of the first dolomitic mudstone bed encountered. L-3 is characterized by a widely fluctuating GR profile, but with a distinct trend of upward-decreasing radioactivity. This reflects the presence of many thin siliciclastic-rich beds and the overall upward decrease in siliciclastic content through the lower part of the unit.

Facies association. - In the lower part of L-3, deposition shifted back and forth between silty dolomitic mudstones, interpreted as hypersaline lagoonal deposits, and deeper-water, bryozoan-rich wackestones. Dolomitic mudstones in the upper part of L-3 tend to be barren with thinly laminated fabric, and include a zone of flat mudstone lithoclasts (2024.4-2025.2 mCD). Frequent surfaces of sharp depositional discontinuity are present, commonly marked by concentrations of small ( < 2 cm) anhydrite nodules. These dolomitic mudstones are interpreted as sabkha deposits.

Cyclicity. - The lower 20 m of L-3 consists of a series of three cycles, each consisting of the following shoaling progression: (l) shale, (2) bryozoan-echinoderm wackestone, (3) dolomitic mudstone. These are viewed as 'catch-down' cycles (Soreghan & Dickinson 1994), formed as relatively deep-water sedimentation was abruptly replaced by very shallow facies in response to falling relative sea leve!. The upper 10 m of L-3 consists mainly of dolomitic mudstone and is interpreted as comprising numerous thin and often highly truncated cycles. In general, the GR log has been used to define major cycles, while sub-cycles have been identified based on the presence of discontinuities inferred to represent extended periods of exposure.

L-4 (late Gzhelian-early Asselian)

Unit L-4 consists of interbedded Palaeoaplysina-dominated buildups and muddy inter-buildup sediments (mainly fusulinid wackestone). L-4 corresponds with the central portion of the regional Barents Sea seismic unit Il of Nilsen et al. (1993). In terms of age and facies, unit L-4 is approximately equivalent with the Kapp Duner Formation of Bjørnøya and the upper Nordenskioldbreen Formation (Tyrrellfjellet Member) of Spitsbergen (proposed changed to Tyrrellfjellet Member of Wordiekammen Formation; Dallmann et al., in preparation).

Facies association. - This unit was deposited in a shallow, variably hypersaline environment, as indicated by the low biological diversity and frequent presence of anhydrite and early dolomite. L-4 Jacks mudstone almost entirely, apparently indicative of deeper water and more favorable biological conditions than under L-3 deposition. Some 21 individual buildups are present, ranging from 0.2-6.7 m in thickness (average 2.2 m), and making up 58% of L-4. These rocks show wide variation in the relative proportions of plates and inter-plate muddy sed-


iment, but are mostly mud-dominated. The platey organisms are in most cases dominantly Palaeoaplysina. Phylloid algae are common to dominant in the buildups below 1992 mCD, but are rare or absent in the upper 56 m of L-4. In buildup facies from central Spitsbergen, a similar, apparently contemporaneous (late Gzhelian) shift is recorded from dominance by phylloid algae in the Cadellfjellet Member to dominance by Palaeo-aplysina in the Tyrellfjellet Member (Johansen & Eilertsen 1995). The inter-buildup sediments in the 7128/6-1 section are mainly fusulinid wackestone, with lesser occurrences of packstone and mudstone. Both buildups arid interbuildup rocks show a wide range in degree of dolomitization, with frequent variations over short vertical distances.

The mud-dominated character of both the buildups and inter-buildup sediments indicates subtidal deposition under quiet-water conditions. Interpretation of relative water depths for the buildup and inter-buildup facies is based on analogy with field studies of the approximately equivalent strata on Bjørnøya and Svalbard, where the buildups commonly stand with several meters of relief above time-equivalent fusulinid wackestone and dolomitic mudstone (Stemmerik et al. 1994; Lønøy 1988; Skaug et al. 1982). These localities display common coalescence of individual buildups into complexes having a relatively flat upper horizon, suggestive of growth limitation by either wave base or maximum tidal range, thus implying shallow water.

Cyclicity. - Two scales of cyclicity are recognizable in L-4. The larger-scale cyclicity is defined by the GR profile and by facies development. These cycles consist of a thinner ( < 1-5 m), high-GR, transgressive zone overlain by a thicker (4-26 m), low-GR, regressive interval. Understanding the depositional geometry of these larger-scale cycles is relevant for hydrocarbon exploration because the transgressive strata tend to have low porosity (mainly 2-5%), and could therefore function as a sealing facies, possibly forming stratigraphic traps where porous regressive deposits pinch out or are truncated.

Five high-GR intervals are present in the 7128/6-1 core (with tops at approximately 2017, 2012, 1995, 1976, and 1953 mCD). The second of these high-GR intervals differs from the others in being very thin; a slow GR scan of the slabbed core surface reveals that this peak is even sharper than shown by the wireline-GR log, and in fact corresponds with a 20-cm bed of laminated, shaly mudstone (2012.55-2012.75 mCD; Appendix l, panel 3). The other high-GR intervals are each 4-5 m thick. The interval at 1995-2000 mCD consists mainly of fusulinid wackestone, but contains a central zone with more open-marine (bryozoan-rich) biota. The two upper intervals (1976-1981 and 1953-1957 m) both contain a basal zone of open-marine to shallow-shelf wackestonegrainstone overlain by an upper zone of subtidal dolomitic mudstone. In well 7128/4-1, the GR log shows


a similar pattern of blocky low-GR zones alternating with thinner zones of irregular high GR activity. Here, however, many of the high-GR intervals are more complex, comprising several well-defined individual peaks. Thin sections of 7128/4-1 sidewall cores from five highGR zones in L-4 and L-5 reveal lithologies consisting of bryozoan-echinoderm wackestone, in several cases with spicule-rich matrix.

The smaller-scale cyclicity in L-4 is defined by alternating buildups and inter-buildup wackestones within the low-GR intervals. Most buildups have a sharp upper contact at which subaerial exposure is a reasonable possibility, based on analogy with outcrop studies of generally similar cycles on Spitsbergen and Bjørnøya (Skaug et al. 1982; Stemmerik & Larssen 1993). Possible evidence for exposure is seen at the tops of several buildups, including truncation of plates at sharp, irregular contacts, infilling of possible cavities in the underlying boundstone by the overlying sediment, and indications of greater dissolution or alteration in the upper 10 cm or so below the contact. These wackestonefbuildup cycles conform to the ideal 'Type 2 platform cyclothems' of Wright (1992), consisting of thick subtidal deposits, little or no intertidalfsupratidal deposits, and a capping exposure surface. According to Wright (1992), this cycle type is typical of Carboniferous 'ice house' climatic conditions, when high-amplitude sea-leve! fluctuations resulted in alternating periods of subtidal 'catch-up' sedimentation and subaerial exposure of carbonate platforms.

L-5 (middle Asselian)

The lower 11 m of unit L-5 consists of wackestone to grainstone (MF-5 and MF-7), including a distinctive ooid grainstone bed at 1930.8-31.3 mCD. The upper 19 m of L-5 is mainly silty dolomitic mudstone, with abundant anhydritejsilica nodules (MF-8). The lower boundary of L-5 is defined by the abrupt termination of the underlying buildup/wackestone cycles.

Facies association. - The mudstone intervals contain many zones with fossils and/or bioturbation, in some cases with fabric bordering on peloidal to bioclastic wackestone, but over half of this section appears barren. Some intervals are thinly laminated, typically due to variations in quartz silt content, but elsewhere with possible algal mat fabric. A probable algal stromatolite with 3 cm relief occurs at 1927.3 mCD. Some samples contain abundant silica sponge spicules, as have been noted to be characteristic of hypersaline lagoonal to transgressive dolomitic mudstones elsewhere (Chowns & Elkins 1974; Goldhammer et al. 1991). Other intervals contain a few vague remnants of corals and shells. The distribution of fossiliferous or bioturbated (subtidal), laminated and mainly non-fossiliferous (intertidal to supratidal), and non-laminated, non-fossiliferous (possible supratidal) fabrics is illustrated in Appendix l. The


MF-8 mudstones are interpretated as shallow, hypersaline lagoonal to sabkha deposits. However, as in unit L-3, indicators of intertidalfsupratidal deposition (fenestrae, mudcracks, algal lamination; Lucia 1972; Shinn 1986) are absent to scarce, so the sabkha interpretation is based largely on the thin-bedded character and predominant absence of fossils.

Anhydrite nodules showing various degrees of replacement by chalcedony and macro-quartz (Maliva 1987) make up 5-10% of the mudstone section. In both units L-5 and L-3, these nodules are of two types. Bulbous nodules 1-7 cm in diameter occur scattered throughout the section, having likely formed displacively at shallow depth. The second type occurs as layers a few cm thick, consisting of tightly-packed mini-nodules several mm to a few cm in maximum dimension. These layers are commonly overlain by or include a shale lamination, and tend to occur at marked discontinuities between overlying and underlying fabrics. Some mini-nodule layers have relief of a few cm, and one case (1915.7 mCD) could be interpreted as displaying enterolithic folding. In general, these features are interpreted as sabkha evaporite deposits, and thus surfaces of extended exposure. However, a few of the nodule layers (1915.35, 2026.65 mCD) have an upward-elongated fabric suggestive of subaqueous growth as gypsum crystals (Schreiber et al. 1982) and therefore may be lagoonal deposits.

An important difference between the L-5 section in the two wells is the presence of 4 anhydrite beds, 0.5-4 m thick, in the 7128/4-1 location, as identified from the density log and cuttings. A fifth anhydrite bed, l m thick, occurs in the lower part of L-6 of 7128/4-1, just below the base of the cored interval. These beds may reflect an overall deeper-water setting in well 7128/4-1, such that anhydrite deposition here was time-equivalent with exposure in the 7128/6- l location. Anhydrite is also abundant in the lower 5 m of L-5 in well 7128/6-1, where it occurs both as a massive cement in packstonefgrainstones, and as concentrations of nodules in laminated mudstone. This tight zone may correlate with the massive, 4 m-thick anhydrite layer recognized from log response at the base of L-5 in well 71 28/4-1. Like the transgressive, high-GR zones in unit L-4 and at 1925-1927 mCD in L-5, the anhydrite 1ayers are of interest for hydrocarbon exp1oration because of their possible ro le as 1aterally extensive sea1s for stratigraphic trapping.

Cyclicity. - As in the underlying unit L-4, two scales of cyclicity can be defined in unit L-5; the larger scale based mainly on the GR profile, and the smaller scale based on litho1ogy and surfaces of sharp stratal discontinuity (Appendix 1). In L-5, five GR-defined cycles may be identified, but the GR peaks marking cycle bases tend to be subtle, and are not associated with obvious clay-rich sedimentologic features. The smaller-scale, lithologicallydefined cycles are interpreted to consist of a basal zone of bioturbated, fossiliferous mudstone (shallow, hypersaline lagoon or restricted shallow shelf deposit), overlain


by barren, silt-rich mudstone (tidal flat to sabkha evrironment), terminated by a layer of cm-scale anhydrite nodules (representing a surface of prolonged subaerial exposure). Probably the best example of such a cycle is the interval from 1922.0-1918.2 mCD, but a general characteristic of these low-accommodation cycles is their incompleteness due to non-deposition and truncation.

L-6 middle Asselian-early Sakmarian

The lower boundary of L-5 is marked at the transition from the underlying interval of anhydrite-rich dolomitic mudstone to the overlying grain-rich limestone interval. Correlation based on the GR profile shows that all but the lower 4.5 m of unit L-6 is represented in the 51 m cored interval of well 7128/4-1. Unit L-6 corresponds with the upper portion of the regional Barents Sea seismic unit Il of Nilsen et al. (1993). In the IKU cores, it is approximately correlative with the Asselian I seismic unit of Bugge et al. (1995), consisting of 50 m of variably dolomitized, shallow-water mudstone to packstone with frequent Palaeoaplysina buildups in its lower portion. Together with the underlying units L-5 and L-4, unit L-6 is roughly equivalent with the upper Nordenskioldbreen Formation (Tyrrellfjellet Member) of Spitsbergen.

Facies association. - The bulk of unit L-6 in both wells is composed of foraminifera- and algae-rich grainstone to packstone. Especially in well 7128/6-l , many of these grainstone/packstones contain abundant red algae plates encrusted by tubular foraminifera, algae, Tubiphytes, and bryozoans. These rocks commonly have a fabric of large plates, up to 10 cm across, either floating in, or, in other cases, binding together a finer-grained bioclastic matrix. Subordinate Palaeoaplysinajphylloid algae buildups vary from 0.2-3 m thick (7 examples in the two wells).

In well 71 28/6-1 , moderate to minor dolomitization is present in the lower third of L-6, but the upper part of the unit is nearly pure limestone. Anhydrite occurs throughout the lower part of L-6 in both wells, mostly in minor amounts, but is absent in the upper part of the unit and in all succeeding strata. The last anhydrite occurrence is 28 m below the top of L-6 in well 7128/6-1, and 4 m below the top of L-6 in well 7128/4-1. Dolomitization is also minor in L-6 in well 7128/4-1, except for extensive early dolomitization in and around two mudstone beds in the Jower third of the unit. Interestingly, there are no traces of dolomite associated with the 1.3-m anhydrite bed at 1839 mCD in well 7128/4-1. The 10 cm of laminated mudstone capping the anhydrite, and patches of mudstone within the anhydrite, all consist entirely of calcite microspar.

Unit L-6 was deposited in an environment of shallowwater sand shoals and tidal channels having greater current energy and lesser accommodation compared with the more protected, deeper-water conditions prevailing


throughout L-4 deposition. Greater biological diversity, lesser dolomitization, and lower anhydrite abundance in L-6 also indicate less restricted conditions than under deposition of L-4.

Cyclicity. - Upward-shoaling cycles composing unit L-6 are widely variable in detail, but in general consist of a basal wackestone, a thicker central zone of grainstone/ packstone, and an upper zone of grainstonejpackstone rich in encrusted and possibly sediment-binding plates of red algae or Palaeoaplysina. Evidence for exposure at cycle tops is merely suggestive, including sharp, irregular contacts with truncation of underlying layering, possible rip-up clasts just over the contact, possible increased dissolution just under the contact, and gradual darkening upward toward the contact. Numerous sharp discontinuities within the grainstone-rich intervals may represent events of either exposure or submarine erosion.

Comparison between wells 7128/6-1 and 7128/4-1. -

Throughout L-6 deposition, overall deeper-water facies development in well 7128/4-l is indicated by a number of observations, including: (l) stronger ·aR peaks in the 7128/4-l profile (Fig. 4), reflecting more pronounced flooding events at this location; (2) greater frequency of wackestone (MF-5) with respect to packstone and grainstone (MF-7) throughout the lower part of L-6 in 7128/ 4-1, as compared with 7128/6-1 (Appendix l); (3) predominance of packstone in the upper half of L-6 in 7128/4-1, as compared with predominance of grainstone in the upper half of L-6 in 7128/6-1 (Appendix l); and (4) occurrence of a subaqueously-deposited anhydrite bed in the upper part of L-6 in well 7128/4-1, which may correlate with one of the postulated exposure surfaces in the 7128/6-1 section.

The underlying units L-5 and L-4 are also interpreted as showing deeper-water facies development in well 7128/ 4-1. As noted above, wire1ine 1ogs and cuttings reveal the presence of several anhydrite beds in unit L-5 and in the lowermost, uncored part of L-6 in 7128/4-1, which are envisaged as correlating to exposure surfaces in the 7128/ 6-1 core. There are a1so more numerous and prominent GR peaks throughout both L-5 and L-4 in well 7128/4-1 (Fig. 4). As described be1ow, the overlying unit L-7 also shows signs of deeper-water deposition in well 7128/4-1 in ha ving much thicker deve1opment of the open-marine facies MF-3 and MF-4 in its cycle 3, and also by the occurrence of Palaeoaplysina buildups higher in the section than in well 7128/6-1. With the exception of unit L-5, overall deeper-water deposition in 7128/4-1 does not correspond with greater stratigraphic thickness (Table 3), indicating that sediment accumulation was not limited only by accommodation, but also by supp1y rate. The postulated difference in water depth cou1d reflect the more 'interior' platform position of well 7128/6-1 (Fig. 2), but could a1so be the result of local-scale relief within what was probably a complex system of persistent shoals and intervening sub-basins extending over much of the


platform. A persistent topographic difference of this nature can possibly be explained by reinforcement of paleorelief due to differential compaction of immediately underlying strata. Relatively emergent areas will tend to resist compaction due to freshwater vadose calcite cementation (Dra vis 1996), whereas areas of moderate) y deeperwater sedimentation will be relatively susceptible to early compaction due to higher mud and shale content.

L-7 (ear/y Sakmarian)

In both wells, the lower contact of L-7 is identified at the first of several beds of calcareous shale, corresponding with two prominent GR peaks in each well. This horizon correlates with the 'base Sakmarian' seismic reflector. Unit L-7 corresponds with the lower portion of the regional Barents Sea seismic unit Ill of Nilsen et al. (1993). In the IKU cores, the lower, shale-rich portion of unit L-7 correlates with the Asselian Il seismic unit of Bugge et al. (1995), consisting of 25 m of calcareous shale. However, the upper portion of L-7, consisting mainly of shallow-water grainstonejpackstone, appears to have no lithologic equivalent in the IKU composite section, indicating either erosion or non-deposition.

Facies association. - L-7 includes a wide diversity of facies (Fig. 6). In the lower part of the unit, beds of silty calcareous shale (MF -9) record a deep-water setting where carbonate production was depressed during a series of three major transgressions. Bryozoan -echinoderm wackestones to grainstones (MF-3 and MF-4) represent apen-marine deposition at intermediate water depths, possibly in the order of 50-150 m. Finally, the upper part of L-7 consists mainly of shallow-water limestones (MF-6, MF-5, and MF-7).

Palaeoaplysinafphylloid algae buildups are absent in L-7 in well 7128/6-1, although there are two thin beds of coral bafflestone and two thin boundstone beds consisting of red and Dasycladacean algae encrusted by tubular foraminifera, serpulids, and Tubiphytes. In wel! 7128/4-1, L-7 contains four buildup beds, the lower two rich in plates of Archaeolithophyllum-like algae, and the upper two dominated by Palaeoaplysina.

In both wells, beds of calcareous, bioclast-rich sandstone occur within the upper part of L-7:

• In well 7128/6-1, sandstone beds respectively 1.0 and 1.5 m thick occur at 1837 and 1849 mCD. In both cases, the sandstone and overlying grainstone are interpreted as making up one cycle, the sandstone recording transgression and the grainstone representing subsequent regression.

• In well 7128/4-1, a sandstone bed forms the top 0.7 m of the cored interval. The thickness to the top of this sandstone is unknown, but it occurs 7 m below the top of unit L-7. It overlies cycle 5 of L-7 and thus has approximately the same position as the lower sandstone in well 7128/6-1, which overlies cycle 4.


These sandstones record sand transport onto the platform during a series of minor sea-level lowstands, followed by reworking during subsequent transgression. It is possible to attach added significance to the upper sandstone in well 7128/6-1 by interpreting this as the basal transgressive deposit formed by southward retreat of the shoreline ahead of the advancing cool-water, storm-dominated environment represented by unit L-8. However, our preferred interpretation is that sand deposition was simply an integral aspect of the cyclic exposurejflooding pattern characterizing the upper portion of L-7, and thus was only indirectly related to the major transgression recorded at the L-7 /L-8 contact.

Cyclicity. - Overall, L-7 shows a pronounced shoalingupward pattern in microfacies and cycle character. The lower three cycles record a major transgression, which took place as a series of large-amplitude sea-level fluctuations. The individual flooding events were pronounced with respect both to depth and probably also duration, producing cycles consisting of a 1ower zone of bioclastrich silty shale, overlain by silty wackestone, and capped by grainstone or boundstone. These three cycles are present in the cored interva1s from both wells. Cycles l and 2 are capped by foraminiferaja1gae-rich lithologies, indicating rapid re-establishment of shallowjwarm-water conditions following each transgression. Cycle 3, however, contains an upper zone of bryozoan-echinodermrich lithology (MF-3 and MF-4), indicating persistence of relatively deep-water sedimentation following this transgression. Cycle 3 is therefore interpreted as recording the maximum transgression in L-7. Overlying cycles consist of shallow-water lithologies, and contain only thin shale layers. Cycles become progressively thinner and less well-differentiated upward, indicating decreasing accommodation and probably therefore diminishing magnitude of sea-1evel fluctuation. In well 7128/6-1, the upper part of L-7 is divisib1e into four cycles, while in well 7128/4-1, only two cycles are apparent. However, in well 7128/4-1, the top boundary of L-7 is interpreted to be 7 m above the top of the cored interval, so additional cycles are probably present.

L-8 (late Sakmarian-late Artinskian)

Unit L-8 is characterized both by very low GR activity almost throughout, and by uniformly low porosity (mostly 3-5%). It consists of bryozoan-echinoderm grainstone to wackestone, with no anhydrite and only traces of dolomite (minor rhombs < 0.1 mm). The lower boundary of L-8 is identified based on a major transition in biota from the underlying foraminifera-algae-dominated (photozoan) assemblage, indicative of warm, shallow-water conditions, to the overlying bryozoaechinoderm-dominated (heterozoan) assemblage, indicative of cooler, deeper-water conditions. This biotic change was an arctic-wide phenomenon reflecting either


climatic cooling or changes in oceanic circulation patterns that introduced progressively cooler waters over these previously warm-water shelves (Beauchamp 1994). In well 7128/6-1, this change takes place across a 3-cm shale bed having sharp contact with the underlying foraminifera grainstone. No lithologic evidence is seen for a hiatus at this contact, such as mineralization, dissolution, or other alteration at the top of L-7. In well 7128/4-1, the boundary is located at 1808.2 mLD, based on: (l) correlation of GR log profiles (in particular, the low-GR 'hump' at 1805 mLD in 7128/4-1 and at 1831 mLD in 7128/6-1), and (2) the contrast in density-log porosity associated with the L-7 /L-8 contact (Fig. 4). This porosity contrast is regarded as a reliable indicator of the L-7/L-8 contact because in well 7128/6-1, tightly calcite-cemented echinoderm-bryozoan grainstones comprising the lower part of L-7 have porosities of 1-3%, whereas foraminifera-alga1 grainstones and packstones in the upper l 0-20 m of L-7 have more variable porosities, commonly in the range 5-12%.

Unit L-8 is equivalent with the upper portion of the regional seismic unit Ill of Nilsen et al. (1993), the exposed equivalents of which are the upper Artinskian Hambergfjellet Formation of Bjørnøya, and the lower Kapp Starostin Formation (Vøringen Member) of Spitsbergen (Fig. 7). The Sakmarian Gipshuken Formation, which underlies the Vøringen Member, could be contemporaneous with unit L-7. In the IKU wells, unit L-8 correlates with a 45-m interval of bryozoan-echinoderm limestone (the Sakmarian-Artinskian seismic unit of Bugge et al. 1995).

Stemmerik (1997) stated that the interval equivalent with L-8 is separated from underlying strata by a regional erosional unconformity and a hiatus covering much of Sakmarian time. However, this is not supported by the information from the present exploration wells. Groves & Wahlman (1997, p. 761) concluded that in well 7128/6-1, 'The 1ower Gzhelian through upper Sakmarian succession is fairly complete but punctuated by hiatuses whose durations are below the reso1ution of fusu1inacean biostratigraphy.'

Roughly 50 km north of the 7128/6 area, the interval corre1ative with unit L-8 contains major bui1dups visib1e on seismic apparatus (up to 250 m in thickness). These bui1dups form an overall arcuate trend (Fig. 2), the location of which is believed to be related to both a paleo-shelf edge along the Nordkapp Basin, and to WNW-ESE basement lineaments (Bruce & Toomey 1993). One of these seismic features was tested by Shell's well 7129/11-1, which penetrated 251 m and cored 160 m of a buildup consisting of Stromatactis boundstone passing upward into bryozoan-cement framestone (Blendinger et al. 1997). North of the buildup trend, the unit passes into muddier, more basinal limestone facies, as seen in well 7228/9-1. According to Nilsen et al. (1993), growth of the buildup trend led to evolution of the Finnmark P1atform from a ramp to a rimmed she1f profi1e. However, an alternative view is that the p1atform


had a distally steepened ramp profile with a marginal buildup trend also in GzhelianfAsselian time (N. A. H. Pickard, pers. comm.). This profile was not significantly altered by growth of a facies belt of discontinuous large buildups during Late Sakmarian transgression, and the buildups were subsequently onlapped and engulfed by progradation of Artinskian bioclastic debris. This transgressive-regressive development corresponds with the first and second of Stemmerik's (1997) three periods of cool-water carbonate platform development, recognized in the Barents Sea and northern Greenland.

Facies association. - Unit L-8 was deposited in a highenergy, open-shelf environment. The biota consists dominantly of bryozoans and echinoderms, with subordinate brachiopods, Tubiphytes, fusulinids, foraminifera, siliceous sponges, ostracodes, gastropods, trilobites, and encrusting red algae. Bioturbation is ubiquitous in the wackestones, and indications of burrowing are common in the packstones and grainstones. Based on analogy with the modem cool-water ( < 20°C) shelves of southern Australia, which produce sediments similar to L-8 (James et al. 1992), the grainstones and packstones may have formed at depths as great as 140 m, although considerably shallower estimates are likely based on the lesser wave energy to be expected in the present intercratonic setting (Stemmerik 1997). Minimal numbers of phototrophic organisms (analogous to coralline algae in the modem example) may indicate depths > 70 m, depending on water turbidity. Grainstones and packstones (MF-4) correspond with Stemmerik's (1997) facies F3, which is attributed to storm deposition below a fair-weather wave base. Wackestones (MF-3) correspond with Stemmerik's (1997) facies F7, which is interpreted to have formed below a storm wave base.

The pronounced contrast in biota with the underlying units (absence of shallow-water buildups and algae; marked reduction in foraminifera) is interpreted to result from a decrease in water temperature. However, biota and lithologies (MF-4 and MF-3) very similar to L-8 occur in thin, transgressive intervals in the underlying units. Transition from warmer- to cooler-water benthic carbonate associations can, in general, reflect either climatic cooling or increase in water depth (Beauchamp 1994). The major and apparently abrupt biotic change at the L-7 /L-8 boundary could therefore have been triggered by a rise in sea leve!. Higher sea level would have tended to eliminate barriers to oceanic circulation, possibly promoting influx of cool-water currents across the platform.

Cyclicity. - Meter-scale shoaling-upward cycles, such as characterize units L-l to L-7, have not been interpreted a bo ve the L-7 /L-8 contact. However, there are frequent alternations between grainstone, packstone and wackestone (Appendix l) and even more frequent stratal discontinuities expressed as stylolitized shale-rich seams (generally on too fine a scale to be represented in Ap-


pendix 1). These lithologic variations reflect fluctuations in current energy that may or may not be regarded as cyclic in character. We interpret these fluctuations as primarily reflecting storm events rather than variations in relative sea level, as in the case of the cycles in units L-l through L-7.

On a much larger scale, the entire unit L-8 represents one transgressivejregressive cycle. There was a gradual increase in water depth from the grainstone-dominated base of L-8 upward to the GR-maximum shale at 1813 m, followed by shoaling to grainstone-dominated deposition throughout the upper three quarters of unit L-8. The lower 21 m of L-8 shows an overall fining-upward pattern, with a lower 5 m of grainstone overlain by 9 m of packstone to wackestone, overlain by 7 m of shaly, silty wackestone having numerous shalejsilt laminations. This trend is traced by increasing GR activity, and culminates in a thin GR maximum at 1813 mCD corresponding with a 65-cm layer of dark grey to black shale containing 25% bryozoan and echinoderm bioclasts. Overlying this horizon, GR activity is monotonously low throughout the upper 68 m of L-8. The lower 23 m of this upper low-GR zone consists of thinly interbedded grainstonejpackstone and spiculitic wackestone, and shows a faint upward-decreasing GR trend. The overlying upper 45 m of L-8 has uniform low GR activity, corresponding to nearly l 00% grainstonejpackstone, with only minor wackestone beds.

L-9 (?Kungurian-Late Permian)

The GR profile shows that unit L-9 comprises two cycles of upward-decreasing GR activity, 58 m thick (cycle l) and 72 m thick ( cycle 2). These cycles reflect tren ds of decreasing siliciclastic content and increasing biological productivity and sedimentation rate, probably due to upward shoaling of the environment. L-9 is sharply overlain by Lower Triassic (Greisbachian) silty shale and siltstone, possibly reflecting onset of the Ural orogeny to the southeast, and uplift of the Norwegian mainland (Johansen et al. 1993).

The base of unit L-9 corresponds with a major change in biota from the underlying 'byonoderm-extended' assemblage to the cold-water 'hyalosponge-bryonoderm' assemblage of Beauchamp (1994). Like the L-7 /L-8 biotic transition, this was also an arctic-wide phenomenon, apparently reflecting further climatic cooling. According to Beauchamp (1994), modem analogues to the hyalosponge (spiculite) assemblage may indicate the development of extensive ice sheets covering the ocean surface.

Unit L-9 is equivalent with the regional Barents Sea seismic unit IV defined by Nilsen et al. ( 1993), the exposed equivalents of which are suggested to be the Miseryfjellet Formation of Bjørnøya and the main portion of the Kapp Starostin Formation of Spitsbergen (Fig. 7). L-9 correlates with a 19-29 m interval that was


cored in two of the IKU wells (the Kungurian-Ufimian and Kazanian-Tatarian seismic units of Bugge et al. 1995). This interval can also be divided into two 'cycles' in the IKU wells, including a thicker (17-24 m) lower unit consisting of chamositic(?) marljsiltstone overlain by siliceous limestone, and a thinner (5 m) upper unit consisting of a 50 cm-thick, high-GR phosphorite bed overlain by spiculite. Based on the GR profiles, it is tempting to correlate these two intervals directly with the two much thicker cycles observed in the exploration wells (Fig. 4). Dating of unit L-9 is based on correlation to the IKU wells, where the lower part of the correlative interval is ?Kungurian-Ufimian, and the upper part is Kazanian-?Tatarian (Mangerud 1994; Bugge et al. 1995). Although erosional truncation at the top of this interval appears likely in the area of the IKU wells, there is presently no compelling evidence either for or against the presence of a depositional hiatus at the top of unit L-9 in the exploration wells.

Drowning unconformities. - The lower boundary of L-9, corresponding with the 'base Kungurian' seismic horizon, is a sharp, irregular surface showing pronounced pyrite cementation and disruption over a zone of 5-10 cm below the contact. Just below the contact, the rock is a brecciated, heavily-pyritized, medium-grey wackestone to packstone with irregular concentrations of bioclasts and glauconite pellets in a micrite matrix cut by irregular veins of coarse calcite cement. Man y bioclasts are heavily bored. Within 15-20 cm below the contact, the rock becomes lighter grey in color and passes into undisrupted grainstone. However, the color continues to become gradually lighter for another roughly 1.5 m downward, until it attains the light grey typical of L-8 grainstone. Immediately above the contact is 4-8 cm of very dark grey shale, overlain by l O . cm of calcareous spiculite (MF-lA), passing upward into the overlying 20 m of dark grey laminated to burrowed calcareous silty shale (MF-l B).

The top of L-8 is interpreted as a 'drowning unconformity' (Schlager 1989 and 1998), based on the abrupt termination of carbonate sedimentation, which apparently occurred throughout the Barents Sea, and the evidence of an extended period of non-deposition and exposure on the sea floor. It may be argued that this is not a true drowning unconformity because strata above and below this contact are parallel, and drowning was followed by eventual resumption of biogenic (silica-dominated) deposition. During the drowning, carbonate production may be imagined to have continued in shallow-water environments landward of the present Barents Sea area, but no record of these settings seems to have been preserved. No evidence has been found of subaerial exposure at the top of L-8, but this possibility is difficult to rule out with certainty.

The top contact of L-9, corresponding with the 'base Tatarian' seismic reflector, may also be a drowning unconformity, in that biogenic sedimentation terminated


abruptly and was then followed by siliciclastic strata having a different (downlapping) stratal geometry. However, it is also possible that this surface was subaerially exposed and represents a major hiatus, insofar as paleontologic dating of unit L-9 is vague. Evidence regarding the possibility of exposure of the taps of L-8 and L-9 is discussed in Ehrenberg et al. (this issue).

Lower cycle. - This interval consists of calcareous silty shale (MF-lB) and calcareous spiculite (MF-lA). Silt and clay content decrease and biogenic silica (spicule) content increases upward through this cycle. The lower 20 m (1725-1745 mCD) has uniform dark grey appearance on the core surface and consists main1y of calcareous silty shale. The upper 24 m of the cored interva1 (1701-1725 mCD, terminating 6 m below the tap of the lower cycle) consists of calcareous spiculite with subordinate silty shale. It has lighter grey color, nodular fabric, and lower GR profile than in the underlying interval. At several places, notably 1715-1716 m, there are 5-30 cm-thick beds of brachiopod-rich, g1auconitic wackestone/packstone, with matrix consisting of either spiculitic chert or quartz and calcite sand. These beds are interpreted as storm deposits. The moderate GR deflection around this interval (1710-1716 m; Fig. 8) apparently reflects the glauconite and clay content of these beds. Most samples from the lower L-9 cycle have TOC in the range 0.2-1.2%, but a few samples (1745 m in 7128/6-1 and 1679 m in 7128/4-1) have 2.0-2.7% TOC, indicating moderate source rock potential.

Upper cycle. - This interval shows differing development in the two wells, as reflected by the less shaly GR profi1e and lesser thickness in well 7128/4-1 (73 vs. 88 m; Fig. 8). Interpretation is hampered by the incomplete core coverage (Fig. 4), but thin sections of sidewall cores and cuttings show that three distinct rock types are present:

l . Silty shale with extremely high GR activity (maximum 220-260 API) forms the 1ower 2-3 m of the upper L-9 cycle. A thin section of a sidewall core from this zone in well 7128/4-1 (1655.50 mLD) consists of silty sha1e with abundant fine-grained sand and roughly 5% chamosite(?) pellets and ooids. Many of the sand grains a1so have concentrically-oriented coatings of clay similar to that composing the ooids. Presence of the chamosite(?) grains supports interpretation of the high-GR zone as a condensed section.

2. Spiculitic limestone forms a zone 53 m thick in well 7128/4-1, but is not present in well 7128/6-1 (Fig. 8). This zone was not cored, but was examined in a series of thin sections from l O sidewall cores. These consist mainly of bryozoan-echinoderm-spicule wackestone to packstone with shale and chert matrix. This lithology is thus similar to MF-3, except for the presence of abundant large (100-500 micron) spicules and the apparent absence of fusulinids.

3. Spiculite composes the entire 70 m of the upper L-9

NORSK GEOLOGISK TIDSSKRIFT 78 (I998)

cycle overlying the high-GR silty shale zone in well 7128/6-1. In well 7128/4-1, spicu1ite forms the upper 21 m of L-9, and also makes up approximately I l m of section underlying the limestone.

The upper part of the spiculite was cored in both wells (Fig. 8), revealing a nodular to lensoid fabric with abundant laminations, and lenses of light grey silty shale (typically green to greenish-grey in the 7128/4-1 core). Burrows and argillaceous pellets are ubiquitous. The spiculite is composed mainly of the chalcedony-cemented molds of monaxon and greatly subordinate multiaxon sponge spicules. Abundant spicules consisting of single crystals of calcite or, in other cases, saddle dolomite, are believed to be primary calcisponge spicules, based on absence of the early silica cements which compose surrounding silica spicules. Whole sponges 2-5 cm in diameter are abundant in some zones. Shell fragments with bored surfaces are common, and whole articulated brachiopods are present in several zones. In the 7128/4-1 core, there is a 5 cm-thick layer of coarse shell debris, interpreted as a storm deposit. In terms of Dunham classification, the spiculite would be packstone with subordinate grainstone, but carbonate bioclast content is typically less than 5%.

Some zones contain abundant clay matrix, although siliceous matrix is dominant in most samples. XRD analysis shows that both the clay matrix and shale lenses consist mainly of illite and chlorite, with subordinate kaolinite in a few cases. The shale lenses are typically rich in silt to fine sand and commonly contain coarse shell fragments, indicating deposition as distal storm events rather than during extended hiatuses. Glauconite pellets and phosphate grains are entirely absent in 7128/6-1, but are common throughout both the spiculite and shale lenses in the 7128/4-1 core. This difference, together with a generally less matrix-rich fabric in the latter occurrence, indicate a shallower depositional setting for the spiculite cored in well 7 1 28/4-1 . Thin sections of sidewall cores and cuttings show that the lower part of the upper L-9 cycle in both wells consists of matrix-rich spiculite with subordinate layers of silty shale.

Spiculite in the upper L-9 cycle was deposited over a range of water depths. The deepest setting is represented by thinly interlayered spiculite and silty shale in the uncored lower part of the cycle, while the shallowest setting, probably near storm wave base, is represented by the upper part of the cycle in well 7128/4-1. In general, the frequent burrowing and light color of the spiculite indicate well oxygenated bottom conditions, while the matrix-rich fabric, abundant shale lenses, low content of carbonate bioclasts, and presence of whole brachiopods indicate deposition below storm wave base. Ubiquitous borings on carbonate bioclasts also indicate slow sedimentation rates and quiet conditions. Similar spiculite of roughly equivalent age in the Sverdrup Basin, Canada (hyalosponge assemblage) is interpreted by Beauchamp (1994) as a shallow-water facies in which biological

NORSK GEOLOGISK TIDSSKR!Ff 78 (1998)

71 28/4-1 o GR (API) 1 50 o Porosity (%) 35 o

I p

- [" {

,,, � --�

�� E l ,, ,

? 'OF ' ;:. _.s--7'

t l ' l <: ;;::: > .... l -

� l , , ',.-'�'

f g � V" �

1 600 <

� c::

� � �

p

l � �

� � � �

> � t '(

l� '77 c;,

� )' \ ,", -�

l<' ,- 1 650 -;;; / !.-

( � �"'-�

1::::-'

E23 Limestone � l..G::.J Spiculite


71 28/6- 1 GR (API) 1 50 o Porosity (%) 35

.( c!= F=-5 -=

� � � - / / .... c:::::

Il "-'

3 OF '"

E " -- ,

It -, ..... � '(\ � l / ' l .._l 1 650 F "..

l �

, -- r---.-�

,- l � � .-'

� , ,� -= � - / >

} 7 1 � =

� , ,

", l

� l / _ ,

} , ' f l 1.!. > .... \

.2 " . ..L = .5

l'> 1 700 � �

<:.

[_ l OF E

� C" �

Fig. 8. Gamma ra y and porosity profiles comparing the upper cycle of 1ithostratigraphic unit L-9 in wells 7128/4-1 and 7128/6-1. Litho1ogy co1umns are based on core

description and thin sections of sidewall cores and cuttings samp1es.

diversity was restricted by factors other than water depth, possibly related to ice cover, low availability of sunlight, and freshwater influx. Shallow-water indicators that are present in the Canadian example but not in the Finnmark Platform spiculite are presence of cross stratification and common admixture of medium-gr_ained siliciclastic sand.

Porous chert reservoirs with similarities to the present example are described by SaUer et al. (1991) and Ruppel & Hovorka (1995) from the Devonian Thirty-one Formation in Texas, where spicule grainstone to packstones are interpreted as submarine fan (turbidite) deposits. Another analog is described by Rogers et al. (1995) from the Lower Carboniferous (Tournaisian) Osage Formation in Kansas, where a lensoid spiculite unit is interpreted as a complex of coalescing sponge mounds or bioherms surrounded by cherty carbonate mudstone to wackestone. In both of these formations, however, the spicules are much smaller (mainly 50 microns or less) than in the upper L-9 cycle (commonly 100-500 microns).

The limestone zone in well 7128/4-1 is suggested to be a bank of locally-derived bryozoan-echinoderm-spicule fragmenta} debris. A possible outcrop analog has been described by Fredriksen (1988) from the approximately time-equivalent Kapp Starostin Formation of Spitsbergen (Fig. 9), where there is a lateral facies transition, over a distance of 3.6 km, from spiculite (9 m thick) into

an interval of bryozoan packstone (19 m thick). A similar model was also illustrated for the Kapp Starostin Formation by Malkowski & Hoffman (1979). Like buildups, these banks are viewed as biostromes of locally derived materials, but lacking the sediment-baffling or binding mode of accumulation characteristic of buildups. The banks are interpreted as having had tens of meters of very gentle relief over the surrounding spiculite-covered sea floor. According to this model, the Upper Permian seismic buildups that have been mapped in the study area (Fig. 2) are interpreted as bioclastic banks whose seismic definition may be enhanced by an overlying zone of porous, partially gas-saturated spiculite. In addition to Fig. 3, seismic images of these features are reproduced in Bruce & Toomey (1993) and Cecchi (1993). Seismic mapping indicates that well 7128/6-1 is located on the flank of the siesmic buildup facies, while well 7128/4-1 penetrated the buildup facies.

Factors controlling development of the carbonatedominated banks, as the alternative to spiculite, are uncertain. The banks are assumed to have nucleated on a spicule-covered seabed in areas of favorable topography or nutrient supply, after which time the localization of carbonate production was reinforced by bioherm relief. Termination of bank growth may have resulted from a rise in relative sea level (as portrayed in Fig. 1 0) or could reflect other types of environmental stress. The


L m

!',1\ l spiculite

� calcareous spiculite


tg bryozoan packstone p:::;::q siliciclastic-rich bryozoan packstone/ � wackestone with shale layers c:::1 calcareous, phosphorous-bearing, t.::.:.::J spiculite with graded layering

Fig. 9. Cross-section based on measured sections at three locations in the lower part of the Kapp Starostin Formation, Akseløya, Svalbard. A bryozoan-packstone

bank passes laterally into spiculite (left). This example is a possible outcrop analog for the upper cycle of unit L-9 in wells 7128/4-1 and 7128/6-1 (Fig. 8). Modified

from Fredriksen (1988).

porous spiculite zone overlying the limestone bank in well 7128/4-1 is interpreted as a relatively shoal-water facies that preferentially colonized topographic highs after abrupt termination of carbonate growth by unknown stress factors.

Sequence stratigraphy Terminology. - The Upper Paleozoic succession of the Finnmark Platform consists mainly of transgressive-regressive, dominantly shoaling-upward cycles of widely varying composition and thickness. These cycles have a characteristic hierarchical pattern, with smaller-scale, 'higher-order' cycles making up larger-scale, 'lower-order' cycles. At all scales; these cycles may be described as depositional sequences, which are defined as being bounded by surfaces or zones of minimum accommodation with respect to sediment supply and are divisible into half-cycles of base-level rise followed by base-level fall (Fig. 11; Cross & Lessenger, submitted). The baselevet rise half-cycle encompasses the lowstand systems tract (LST) and succeeding transgressive systems tract (TST) of Handford & Loucks (1993), while the base-level fall half-cycle is equivalent to the highstand systems tract (HST). In general, the TST and HST are separated by a maximum flooding surface (MFS). In the interior platform setting of the present well study, the LST deposits ideally initiating each sequence are generally absent, being replaced by exposure surfaces.

Sequences and their component systems tracts are defined in terms of stratal geometry and are therefore

best studied on shelf-to-basin transects where the critical geometrical relationships can be observed (Handford & Loucks 1993). However, in the present one-dimensional study, we attempt to infer these relationships based on vertical successions of facies, a more uncertain but nevertheless widely popular basis for sequence recognition. Our interpretation of the sequence stratigraphic organization of the Moscovian-Upper Permian platform succession is summarized in Fig. 4. We identify seven main sequences, designated S-1 through S-7. As described below, these sequences are components of yet larger-scale (roughly 30-60 million year) cycles, and the first four sequences are made up of smaller-scale cycles (1-25 m thick and generally <l million year duration). An important characteristic of the seven main sequences is that they are the smallest cycles potentially resolvable for study on seismic lines.

Following Goldhammer et al. (1990), sequences having duration on the order of 1-1 O million years are regarded as 3rd-order, while smaller-scale sequences are viewed as products of 4th- to 5th-order fluctuations in relative sea level. However, due to the rather loose age constraints (Table 4), it is uncertain if our seven main sequences should be classified as 3rd-order or as lower-order 'sequence sets' containing component 3rd-order sequences. These seven sequences appear to be comparable in scale with the Cenozoic 2nd-order 'megasequences' (roughly lO million year duration) described by Eberli & Ginsburg (1989) from the Great Bahama Bank, while some of the major cycles within sequences S-1 through S-4 resemble the 3rd-order Bahama sequences. Some of these major


7128/4-1 o

7128/6-1 o

/ ; ' '

/ .... .... / ' ' ' ; ' / ;

\ / ; ' / ;

' / ....

.... - ;

-----�-------=-------------------- --�---_-_ __ __ __ __ __ __ ______ __ ____ ---- - - -- - -

l Shale and siltstone 1�-.-s l Shaly spiculite

Il Bioclastic cherty � Condensed sedimentation limestone bank

-- (high-GR silty shale)

l / .... . \ /' l High-porosity spiculite

V Drowning unconformity

(shallower-water facies or leached) (sequence boundary/MFS)

l/ ..... \;' l Low-moderate porosity spiculite Base-level fall (possible lateral sealing facies)

Iso-time surfaces

Fig. JO. Depositional model explaining lateral and vertical facies variations in the upper cycle of unit L-9. Transition from carbonate bank to porous spiculite in the

upper part of this cycle in well 7128/4- 1 is attributed to a rise in relative sea leve!, followed by a depositional hiatus on topographic highs and eventual colonization

by a relative! y shoal-water sponge community.

cycles are also similar in scale to the extensively studied Upper Carboniferous cyclothems of the central USA (Heckel 1977), which are variously regarded as 3rd-order sequences by some workers (Ross & Ross 1995) and 4thto 5th-order sequences by others (Heckel 1986; Youle et al. 1994).

The present sequence stratigraphic scheme may be compared with the interpretation presented by Stemmerik et al. (I 995) for the IKU shallow co res covering Gzhelian-Asselian strata roughly equivalent with units L-4 through L-6 (sequences S-3 and S-4) of the present study. They defined six '3rd-order' sequences, as shown in the porosity column of the composite IKU wells profile in Fig. 4. However, sequence stratigraphic interpretation of the IKU composite section is beyond the scope of the present report.

Time durations. - Paleontological age determinations (Fig. 4) indicate that the absolute time durations repre-

sented by our seven main sequences are on the order of 1-20 million years (Table 4; Fig. 7). However, these values are very uncertain, because there is generally a possible range of several million years in transforming the paleontological dates to an absolute age using the time scale of Harland et al. (1990). For example, 'early Kasimovian' might be interpreted as being anywhere from the basal Kasimovian boundary (303 Ma) to half way through Kasimovian time (299 Ma). The leve! of this uncertainty is indicated in the right-hand column of Table 4. Additional uncertainties derive from varying degrees of confidence in the paleontological datings at different places in the stratigraphic succession and divergences on the order of 5-15 Ma in alternative versions of the absolute time scale (Harland et al. 1990; Klein & Beauchamp 1994; Menning 1995). However, the greatest source of uncertainty is probably the unknown fraction of the total time span contained within surfaces of exposure or condensed sedimentation. As described by


Eustasy

Accomodation Rate +

Sediment Supply Rate

A-R.- 8-S.R. +

LST

MFS

TST

base-leve l rise

----- ----

base-leve l fall

Fig. Il. Conceptual model for sequence stratigraphic terminology applied to the

interior portion of a carbonate platform experiencing large-magnitude eustatic

sea-leve! changes, no tectonism, and gradual subsidence. For simplicity, only a

single order of sea-leve! variation is shown, and the platform interior is assumed

to be exposed througbout LST time. Rate of accommodation creation (eustacy

plus subsidence) is zero during exposure; instantaneously attains a positive value

as the platform is flooded; increases as the slope of the eustacy curve increases

during transgression; attains maximum value corresponding to the MFS; de

creases as rate of sea-leve! rise slows; and continues to decrease as sea leve! begins

to fall (but remains positive due to subsidence component). Rate of sediment

supply is shown divided into siliciclastic and carbonate components. Siliciclastic

supply is greatest during exposure and decreases to minimum at MFS. Carbonate

supply rate is negative during exposure (due to dissolution); increases abrupt! y

with arrival of transgressive shoreline; decreases with increasing water depth; and

rapidly increases to maximum plateau value with HST establishment of carbonate

factory. Difference between accommodation rate and sediment supply rate defines

base-leve! cycles (sequences).

the fractal model of Plotnick (1986), hiatuses may contain a large and unevenly distributed proportion of the total depositional time recorded by paleontologic markers. Hiatuses are likely to be located at sequence boundaries and maximum flooding surfaces, but available dating constraints are largely inadaquate to evaluate their occurrence.

High-frequency cyclicity. - The cycles within sequences S- l through S-4, including both 'major cycles' and component sub-cycles, may be grouped into six general types (Fig. 12), although individual cycles within each category show wide variations (Appendix 1). All these cycles originate from trends of decreasing water depth during


Table 4. Age constraints and corresponding minimum and maximum durations

(Harland et al. 1990) of major sequences composing the Upper .Paleozoic succes

sion in wells 7128/6-1 and 7128/4-1.

Duration (m.y.)

Sequence Age at sequence top (Ma) min.-max.

S-7 Late Permian (256-245) <1-15

S-6 ?Kungurian (260-256) <1-9

S-5 late Artinskian (265-260) 10-22

S-4 early Sakmarian (282-275) 2-13

S-3 m. Asselian (288-284) 2-10

S-2 late Gzhelian (294-290) 4-10

hiatus late Moscovian-m. Kasimovian 3-9

S-1 late Moscov. (307 -303) <1-4

hiatus Brigantian-late Moscovian 25-33

S-0 Brigantian (336-332) 3-11

base S-0 Holkerian (343-339)

deposition, followed by relatively abrupt increase in water depth.

Three types of processes may control these repetitive oscillations: ( l ) eustatic sea-level fluctuations (Goldhammer et al. 1990; Calvert et al. 1990); (2) local tectonic base-level adjustments (Johannessen & Steel 1992); and (3) autogenetic processes (Satterley 1996; Michalzik 1996). Insofar as the present strata were deposited during a time of high-amplitude, high-frequency sea-level fluctuations characterized by finely correlative cyclic sedimentation world-wide (Ross & Ross 1995), it is to be expected that the cyclicity observed in the Finnmark Platform strata contains a major eustatic signal. Tectonism could also be important, despite the apparent lack of faulting within the Finnmark Platform in Upper Carboniferous and Permian times. As noted above in connection with Fig. 3, the pattem of thinning and individual truncation of platform units toward the Norwegian mainland may partly reflect episodic hinge-line tilting that could have had major influence on depositional cyclicity.

Key empirical evidence necessary for demonstrating extemal (sea-level) control on cyclicity is the presence of exposure surfaces at cycle tops (Schlager 1993) and the lang-range lateral persistence of individual cycles (Montanez & Read 1992). Exposure surfaces, although mostly based on inconclusive criteria, are inferred to occur at numerous horizons in the studied section (Appendix l). Conclusive evidence for exposure surfaces capping similar cycles is abundantly present in approximately ageequivalent carbonate strata outcropping on Spitsbergen and Bjørnøya (Pickard et al. 1996; Stemmerik & Larssen 1993). Lang-range correlation of our individual cycles is also generally inconclusive, but with two important exceptions: ( l ) the major cycles in unit L-4 (type 3 in Fig. 11), where transgressive, high-GR intervals appear to be directly correlative between the two wells studied (Fig. 4), and (2) the shale-to-grainstone cycles making up the lower part of unit L-7 (type 6 in Fig. 12). The latter cycles not only appear individually correlative between wells, but also involve greater variations in water depth


1

• • • o • • • • o • • o • • • • o • • • • • • o . o • • . . . . . . . o • • • o • • • • o • • o • . . . . . . . .... . . . • o • • • • • .. . . . . . • o • • o • • • • • o • • • • • • • • o o • • • o . o • • • • • • • o • • • • • • o . . . . .. . . . . . . . . . . .. . . .

LIMESTONE l SANDSTONE

Top = limestone & calcareous sandstone (MF-3, -5, -6, -7, -11), contalning multiple sub-cycles capped by exposure surtaces

Base = dolomitic sandstone (MF-11), overtylng dolomitic mudstone (MF-8) & shale (MF-9)

12 Sl ] . . . . .

3


DOLOMITIC MUDSTONE Top = barren to laminated dolomitic mudstone &

anhydrite (MF-8), capped by exposure surface

Base = fossiliferous/bioturbated dolomitic mudstone (MF-8)

(Cycles typically truncated or partly developed.)

GRAINSTONE l PACKSTONE

Top = foram/algal grainstone/packstone (MF-7), commonly capped by thin buildup (MF-6) and exposure surface

Base = wackestone/packstone (MF-5, -7)

WACKESTONE l SIL TSTONE l SHALE Top = siHy wackestone (MF-3) /calcareous

siHstone (MF-10)

BUl LD UP l WACKESTONE

Base = siHy shale (MF-9)

Top = sub-cycles of Pa/aeoap/ysina l phylloid algae bulldups (MF-6), capped by exposure surface, overtying basal fusulinid wackestone (MF-5)

Base = shaly (high-GR) fossiliferous/bloturbated dolomitlc mudstone (MF-8), fusulinld to bryozoan wackestone (MF-5, -3), & grainrich shallow-water IHhologles (MF-7, -11)

GRAINSTONE l SHALE

Top = gralnstonelpackstone (MF-3, -4, -5-7), some cycles capped by thin bulldup (MF-6)

Base = calcareous shale (MF-9)

Fig. 12. Representative examples of six different cycle types from the core log of well 7128/6-1 (Appendix 1). GR profile is shown on left. Lithology symbols as defined

in Appendix l. Tops and bases of cycles and sub-cycles are shown by arrows at right.

than can reasonably be accounted for by an autocyclic mechanism. The more major high-frequency cyclicity within the Finnmark Platform succession is therefore inferred to reflect mainly relative sea-leve! fluctuations having a major eustatic component, but possibly also driven by episodic tilting. The finest scale of cyclicity could have a greater degree of autogenetic control, but this can only be evaluated by analogy with detailed studies of age-equivalent outcrop sections (Pickard et al. 1996).

Fischer p/ots. - Fig. 13 presents Fischer plots (Sadler et al. 1993) for sequences S-1 through S-4. In the strictest sense, this graphical device is intended for application to cycles of roughly uniform depositional duration with peritidal capping facies, whereby it is envisioned to display an approximate record of relative sea-leve! fluctuations. Clearly these conditions are not met by the present strata, where there is a wide diversity of cycle types present, man y of which may be capped by exposure surfaces, but few of which contain peritidal deposits. Fischer plotting of these cycles is therefore intended only to illustrate patterns of variation in cycle thickness and thereby the overall pattern of minimum accommodation creation through time. U se of this device is not intended to carry the implication that cycles are of similar absolute duration.

On the contrary, we admit considerable uncertainty regarding the most appropriate scale of cyclicity to be compared from ane depositional interval to another. Thus in Fig. 13A, only larger-scale cycles are shown (corresponding with the 'major cycles' of units L-1 through L-5, but including all cycles in units L-6 and L-7), while Fig. 13B includes all finer-scale 'sub-cycles.' The shape of the cumulative curve is rather different on these plots, but this difference is more superficial than fundamental in character. In the following discussion, these plots serve as a convenient reference for comparing the widely variable characteristics manifested by the individual sequences. Cycle thicknesses have not been corrected for the greater compaction of shale than carbonate, so it should be noted that the relative thickness of shale-based cycles (types 2 and 6 in Fig. 12) was originally much greater than is shown by Fig. 13.

Sequence S -1 The shallow-water carbonatejsandstone cycles of unit L-1 are the upper portion of a series of marine beds infilling the block-faulted topography developed during the mainly preceding tectonic episode. The seismic unit


cycle types:

A =11. LIMEST. : SANDST.

E V

E o Ul L U)

Q)

w c L. .Y. :l u

� .c a.+' w u Q)

w > u

A u

o

c

80

70

60

50

40

30

20

:l o 1 o E :l Q)

u E o

8 40

E V

E 30 o Ul L U)

Q)

w c L. .Y. :l u

� .c a.+' w

u Q)

w > u

A u

o

c :l o E :l Q)

20

1 o

o

-10

u E -2 o

S-1

o

S-1

L-1

o

"

5

"

o

12. WACKEST. SILTST. SHALE

S-2

1 o

L-2

20

B 13. BUILDUP WACKEST.

S-3 B

5 20

,4. DOL. MUDST.

25 30


15. GRAINST. PACKST.

S-4

35

� 6. GRAINST. i SHALE

40 45

C y c l e n u m b e r

S-3 S-4

30 40 50 60 70 80 90 100 Cycle n umbe r

Fig. 13. Fischer plots showing pattems of cycle thickness and type in wells 7128/6- 1 and 7128/4-1. See Sadler et al. (1993) for explanation of Fischer p1ots. Vertical

segments of the saw-tooth plots represent thickness of individual cycles, whereas diagonal segments are all of equal length, with the vertical component equal to average

cycle thickness. Pattems of vertical segments represent cycle types defined in Fig. 12. The plot for the 7128/4-1 core was made using the average cycle thickness

determined for 7128/6-1 and is inset over the longer 7128/6-1 plot, correlated to L-6/L-7 boundary. A) Only major cycles included. B) All sub-cycles included.


equivalent with L-l thins over the crests of fault blocks, and is therefore likely to contain major erosional or non-depositional gaps in the well locations, as is also suggested by the pronounced thinning of unit L-1 from well 7128/6-1 to 7128/4-1 (Fig. 4).

The base of S-1 is recognized as an erosional unconformity on seismic, while the top surface appears locally unconformable; both are therefore viewed as type l sequence boundaries. The lower boundary is overlain by a 13 m conglomeratic sandstone, possibly representing an incised valley fill and thus the only LST deposit recognized in this platform succession. The first prominent GR peak overlying this level is tentatively identified as the MFS, such that most of the strata present are assigned to the base-level-fall hemicycle (HST).

Sequence S-2

Deposition of unit L-2 reflects continued major clastic infl.ux onto the platform, but in a distinctly deeper-water setting than in the underlying unit L-1. The lowermost cycle is the thickest and most pronounced; it culminates in shallow-water carbonate facies. It is succeeded by progressively thinner cycles having similar facies association, but which failed to fill their accommodation space to produce shallow-water cycle tops. Decreasing accommodation is indicated at the base of unit L-3 by the first appearance of anhydrite-bearing dolomitic mudstone, interpreted as shallow, hypersaline lagoonal to sabkha deposits. Through the lower 20 m of L-3, deposition switches back and forth between shallow hypersa1ine conditions and the previous open-marine environment, defining a series of three cycles, each consisting of the shoaling progression: (l) shale, (2) bryozoan-echinoderm wackestone, (3) dolomitic mudstone. The culmination of this regressive trend is reached in the interval of thinly bedded, silty dolomitic mudstones in the interval 2018-2027 mCD, where multiple exposure surfaces are interpreted. Overlying deposits are dominantly subtidal buildup and wackestone facies, representing deeper-water conditions. The sequence boundary is therefore interpreted as being somewhere in the upper part of the dolomitic mudstone interva1; at approximately 2024 mCD. The MFS is suggested to correspond with the GR-maximum at the base of L-2, such that virtually all of S-2 consists of HST deposits.

Sequence S-3

This sequence begins at the 'tumaround point' from overall decrease to increase in water depth and cycle thickness within the dolomitic mudstone interval at 2018-2027 mCD. Subsequently, deposition progressed into a thick aggradational succession of fusulinid wackestonejmud-rich buildup cycles. As diagrammed in Appendix l and Fig. 1 1 , these wackestone/buildup cycles comprise relatively thick, low-GR (high-stand) intervals


that altemate with thinner, high-GR (transgressive) intervals, forming a series of six high-order (possible 4th-order?) sequences in units L-4 and L-5 of well 7128/6-1 (Fig. 4). Most of these can be correlated with minimal ambiguity to well 7128/4-1, where the bounding GR peaks are both more pronounced and more complex, indicating overall deeper-water conditions (Fig. 4).

Overlying L-4, there is an abrupt shoaling of the depositional environment, with 11 m of shallow-shelf grainstonejpackestone. This is followed by a second shoaling step at the contact (1924.7 mCD) with the overlying 19 m of restricted-lagoonaljsabkha mudstones. The appearance of thin buildup beds near the top of this interval (1910-1912 m) marks the first sign of an episodic deepening of the environment, leading to deposition of the overlying shallow-shelf grainstonejpackestones of L-6. As in S-2, therefore, the sequence boundary is interpreted as being located at the 'tumaround point' of the overall regressional development. This would be located somewhere in the upper part of the dolomitic mudstone interval, approximately 1913 mCD.

The MFS for sequence S-3 is difficult to define, although there are six distinct fl.ooding events in the 7128/6-1 section, corresponding with prominent high-GR intervals. The key fea ture of the TST-HST boundary is that it represents the turnaround from increasing to decreasing rate of accommodation creation with respect to rate of sediment supply (Schlager 1993; Cross & Lessenger, submitted). This point is classically, but not necessarily expressed as an MFS. During a period of rising 3rd-order relative sea level, rapid sediment production may keep pace with accommodation creation, resulting in an aggradational interval of thick cycles (as seen in L-4) with no net retrogradationa1 displacement of facies tracts upward through the section. As the 3rd-order sea-level rise slows, entering the high-stand phase, the result will be an abrupt filling of accommodation space and seaward displacement of facies tracts (as seen at the L-4/L-5 contact). Using this logic, unit L-4 is identified as the base-level-rise hemicycle (TST) of sequence S-3, with unit L-5 forming the base-level-fall hemicycle (HST).

Sequence S -4

This sequence includes the top 7 m of L-5 and all of units L-6 and L-7. The base-level rise hemicycle of S-4 shows the following retrogradational development in well 7128/ 6-1 : (l) 7 m of subtidal dolomitic mudstone (restricted lagoonal setting); (2) 38 m of aggradational foraminiferaalgal packstone/grainstone cycles (sand shoals); (3) a series of three major transgressional pulses (major eustatic sea-level fl.uctuations), culminating with the deposition of shale and glauconitic wackestone in cycle 3 of unit L-7, where the MFS is placed. The thick sand-shoal package (unit L-6) has a subtly retrogradational character in both wells, as indicated by:

• more frequent minor GR peaks and thicker buildup beds in the upper half of L-6;


• dramatic upward porosity decrease (Fig. 4); • upward decrease in early dolomitization (Appendix

1). Our interpretation of this interval as the aggradational response of the inner platform to rising base-level in sequence S-4 is analogous with our interpretation of the aggradational unit L-4 in sequence S-3. However, the difference is that in S-4 there was a major acceleration of base-leve] rise producing transgression, while in S-3 there was not.

The base-level fall hemicycle of S-4 (upper portion of L-7) shows a strongly progradational character, recordmg the re-establishment of warm, shallow-shelf, foraminiferafalgae-rich conditions. The sequence boundary is placed at the top of L-7, where there is a marked change in biota but not texture (grainstone having similar particle size both above and below the contact). This change is interpreted as reflecting a relative sea-level rise, resulting in increased water depth, but with similarly high-energy (storm-dominated as opposed to shallow-shelf) conditions. This point marks the termination of shallow-water, foraminiferajalgae-rich biota on the platform. Overlying units contain only open-shelf to deep-shelf fauna dominated by bryozoans, echinoderms, and siliceous sponges, and all subsequent sequence boundaries are drowning unconformities.

Sequence S-5

During the initial part of S-5 deposition, gradual deepening resulted in transition from grainstonesfpackstones (1834.7-1826.3 mCD) upward into 13 m of wackestone beds (1826.3-1813.6 mCD). Maximum flooding is marked by a thin GR maximum at 1813 mCD associated with a 65 cm shale bed. Fusulinid dating indicates that this shale corresponds to the Sakmarian/ Artinskian boundary (V.L Davydov, unpublished). This was followed by a thick (68 m) upward-shoaling trend (HST) consisting mainly of bryozoan-echinoderm grainstone. A sequence boundary at the top of L-8 is indicated by the abrupt contact with overlying black silty shale, interpreted as a drowning unconformity. It is unknown whether this surface was subaerially exposed, at least in the area of well 7128/6-1. No evidence of truncation is seen on seismic, and stable isotopic analyses (Ehrenberg et al., this issue) reveal no indications of increased meteoric signature approaching the top of unit L-8.

The transition from photozoan (sunlight-dependent) to heterozoan (mainly sunlight-independent) biota at the L-7 /L-8 boundary is suggested to reflect the beginning of a gradual rise in relative sea level and the resulting onset of more open circulation over the platform. The concomitant termination of meter-scale cyclic deposition at this boundary could reflect both increased water depth and decreased magnitude of high-frequency sea-level fluctuations. Both transgression and damping of sea-level


fluctuation coincide with and are plausibly resultant from the termination of global ice-house conditions in mid-Sakmarian time (Veevers & Powell 1987). Similar and approximately synchronous changes in carbonate deposition and biota also occurred in both Svalbard and Arctic Canada (Stemmerik & Worsley 1989; Beauchamp et al. 1989a; 1989b ).

Sequences S-6 and S-7

As discussed above, these two cycles of upward-decreasing shale content probably reflect gradua1 upward decrease in water depth. It is uncertain whether the upper boundaries of either S-6 or S-7 were subaerially exposed in the study area, although indications of exposure are reported in the much thinner correlative intervals cored in the IKU wells (Bugge et al. 1995). Terminal shutdown of organic platform sedimentation and influx of siliciclastics at the top of S-7 is an arcticwide event, related to the Ural Orogeny farther to the east (Johansen et al. 1993). This transition from biogenic to siliciclastic sedimentation becomes younger westwards, from Early Permian adjacent to Novaya Zemlya to Late Permian in the Norwegian Barents Sea (Dore 1991).

2nd-order sea-leve! trends

2nd-order (roughly 30-60 million year duration) cycles may be interpreted from trends in overall water depth shown by successive sequences. Prior to establishment of the carbonate platform, Visean siliciclastic strata record a trend of upward-decreasing grain size and increasing marine influence terminated by a major unconformity. This transgression is related to the mid-Carboniferous rifting episode recognized throughout the Barents Sea (Stemmerik & Wors1ey 1989).

Following a major hiatus, renewed transgression submerged crests of fault blocks in Moskovian time and culminated in Kasimovian time with the deep-water shales deposited in unit L-2 (Fig. 4). The succeeding sequences S-2 and S-3 show progressive shoaling to the 2nd-order accommodation minimum at the top of S-3. Thick evaporite deposits of the Nordkapp basin probably formed mainly during this 2nd-order Iowstand, although lower levels of these evaporite deposits are probably correlative with various preceding lowstand events, including the exposure surfaces inferred to be present within units L-4, L-3, and L-1.

Subsequently the platform experienced progressive, stepwise base-level rise, at ]east to the drowning unconformity terminating L-8. Despite the clearly deeper-water setting of L-9 as compared with L-8, we interpret the L-9 /L-8 contact as marking the beginning of a new 2nd-order cycle of base-level fall because of the overall upward-shoaling character of L-9 deposition. The 2nd-


order pattern depicted in Fig. 4 may be compared with analogous results of Stemmerik & W orsley (1989; their Fig. 11). A similar, approximately time-equivalent evolution of sequence characteristics and lithologies also took place in Arctic Canada (Beauchamp et al. 1989a, 1989b). More recently, Stemmerik (1997) proposed a somewhat different model of 2nd-order depositional sequences that includes the Finnmark Platform succession. His model groups the present sequences S-l through S-4 as one 2nd-order sequence; S-5 as another; and S-6 and S-7 as a third 2nd-order sequence. This model rests in part on the interpretation of a major hiatus and unconformity at the base of S-5, which, as discussed above, is not evident in the present exploration wells. Pending more careful study, we suggest that the model diagrammed in Fig. 4 is also consistent with regional data in the western Barents Sea, while the correlations to northern Greenland are relatively speculative.

Conclusions Moscovian through Upper Permian strata in the two exploration wells studied record the changing interplay of climate, siliciclastic influx, and 5th- through 2nd-order fluctuations in relative sea level over 50-60 million years of carbonate platform evolution. Nine lithostratigraphic units are defined based on facies associations and are correlated with regional seismic units and outcrop sections on Svalbard.

Only a minor fraction of the carbonate strata are peritidal facies, implying that the platform surface oscillated between periods of subtidal depositional 'catch-up' and subaerial exposure throughout its initial 30 million years (late Moscovian through Early Sakmarian), followed by deeper-water 'catch-up' deposition in its remaining 20-30 million years (late Sakmarian-Late Permian). Well 7l28/4-1 is interpreted as showing overall deeper-water facies development than well 7128/6-1 (25 km to the west) from Late Gzhelian through Early Sakmarian time. This persistent topographic difference can possibly be explained by reinforcement through roughly 19 million years of depositional time by differential compaction of underlying strata.

Seven major depositional sequences are recognized based on trends in facies and cycle thickness. Wide variations in sequence boundaries and internal composition are interpreted in terms of varying rates of higherorder sea-level change and varying position with respect to the inferred 2nd-order sea-level trend. The present one-dimensional study is proposed as a working model in need of further evaluation using seismic data, regional well correlation, and more detailed biostratigraphic control.

Acknowledgements. - Our work has been aided by frequent reference to the

unpublished 1992 sedimentologic study of the 7128/6-1 cores by Ra y Mitchell of

Conoco. Da ting of Visean and Triassic strata is based on unpublished palynology


studies performed for the licenses, with the most specific determinations provided

by an unpublished 1994 report by D. McLean, University of Sheffield. Age data

for the post-Visean section (Fig. 4 and Ta ble 3) are based mainJy on fusulinid

dating summarized in Groves & Wahlman (1997) and Wahlman et al. (1995), by

Inger Nilsson of Saga Petroleum a/s, and by V. I. Davydov of the Permian

Research Institute, Idaho, U.S.A. The manuscript was improved thanks to

suggestions from Nei! A. H. Pickard, Inger Nilsson, Frode Hadler-Jacobsen,

Knut Kirkemo, Wolfgang Schlager, and NGT referees Marcello Cecchi, Snorre

Olaussen, and Anthony M. Spencer.

Manuscript received February 1996

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Wahlman, G. P., Davydov, V. l. & Nilsson, l. 1 995: Fusulinid biostratigraphy

of subsurface cores from the Conoco 7 1 28/6- 1 well, offshore Barents Sea,

Arctic Norway: XIII International Congress on Carboniferous-Permian (XIII

ICC-P), Abstracts, Panstwowy Instytut Geologiczny, Krakow, p. 1 50.

Warren, J. K. 199 1 : Sulfate-dominated sea-marginal and platforrn evaporite

settings: sabkhas and salinas, mudflats and salterns. In Melvin, J. L. (ed.):

Evaporites, Petroleum and Mineral Resources, 69- 1 87. Elsevier, Amsterdam.

Wright, V. P. 1992: Speculations on the controls of cyclic peritidal carbonates:

ice-house vs. greenhouse eustatic controls. Sedimentary Geo/ogy 76, 1 - 5.

Youle, J. C., Watney, W. L. & Lambert, L. L. 1994: Stratal heirarchy and

sequence stratigraphy - Middle Pennsylvanian, southwestern Kansas, USA.

In Klein, G. D. (ed.): Pangea: Paleoclimate, Tectonics, and Sedimentation

During Accretion, Zenith, and Breakup of a Supercontinent. Geological Soci

ety of America Special Paper 288, 267-285.

is shown as a dashed curve. 'CCD' = 100 calcitef(calcite + dolomite) determined

by bulk XRD analysis. 'SCI' = Siliciclastic + Chert Index (defined in text). Width

of horizontal bar pattern to right of SCI field indicates weight % anhydrite

(calculated from bulk sulfur analysis). Width of lithology column indicates

Dunham classification for carbonates and grain size for siliciclastics. Patterns

indicate microfacies category. A model for the relationship of these microfacies to

water depth and position on platforrn depositional profile is illustrated on the

next page.

2 1 4 S . N. Ehrenberg et al. NORSK GEOLOGISK TIDSSKRIFT 78 (1998)

- MF-1A

- MF-1 8

1·;/ ·>l MF-2

Calcareous fine-grained spiculite (subordinate silty shale)

Calcareous silty shale (subordinate fine-grained spiculite)

Coarse spiculite

� � � � [0/1 �

Bryo2;oan-echinoderm wackestone

Bryozoan-echinoderm grainstone/packstone

Fusulinid/foraminifera wackestone/packstone

w � Buildups (plotted in "boundstone column", but including both boundstone and clastic fabrics) . Dominant baffl ing/binding organisms:

@ Palaeoaplysina � Codeacean phyl loid algae $ Archaeolithophyllum-like phylloid algae ® Encrusting forms (tubular foraminifera, algae, etc.) © Coral

[[[I] ME:Z Foraminifera grainstone/packstone

.M.E:fl Dolomitic mudstone:

Ir----,1 barren

- laminated

containing bioturbation and/or bioclasts

- � Shale & silty shale (Lithology column width indicates silt content.)

1::;:;:;:;:] ME:1Q Calcareous siltstone

Q ME:11 Sandstone

b""\\:\,._1 M.E:12 Anhydrite

o 0 Ooids

Glauconite: * minor ** abundant

---------- Shale layers (mainly 1 -5 cm thick)

�>------<� Layers of srriall (<1 -2 cm) anhydrite nodules

Contacts between different l ithologies: hodzontal line = sharp, d iscontinuous .DQJin.e = gradational

L-1 Lithostratigraphic unit boundary

� Sequence boundary

deep shelf apen-marine shelf

periodically restricted sand shoals shallow shelf restricted

lagoon sabkha

MF-4 MF-5 MF-6 MF-7 MF-8 M F-9 MF- 1 0 MF- 1 1 MF- 1 2

m æ � � m � • EillJ D [22]


Appendix 1, panel 1 of 10

Gamma ray <A Pil

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Finnmark carbonate platform, Barents Sea 21 5

7 1 2 8/ 6 - 1 Porosity (1.)

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216 S. N. Ehrenberg et al. NORSK GEOLOGISK TIDSSKRIFT 78 (1998)

Appendix 1, ponel2 of 10 7128/6-1

L-2

Lithology � � Gamma roy _, ., Porosity CCD SCI • <API) Coro "' "'"' • s. � < Cl.l anhydrite ldopth l l l l U 6 1 •• , 15cl cin> 1�'' r � r l� D!lls ��-· ��i

*

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1-+--t--+--+-+-:#, -� 14 le lo lo lo 12 r----·JI4 12 13 �+44-�l�++-*� lelo lol4

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r1C 10 10 !4 -9 lo 1o 14 -9 1o 1o 14 -9 lo 14

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Appendix 1, panel4 of 10

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8/6-1 Lithology � !

' 'Sh"""'=·�-' Sl .. CAPI) Coro _,. 1 1 � Gamma roy ' ��· l "' Porosity

(/.) CCD SCI• l i

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ill

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1-H+ttttt+ftl-tttt++ttt 7[1 121214 l o [1 1-+-+++1 ':;;��o'' l+t+++t+++�t++++++ 1[1 1313[4 1 o lo [o

H----i-+++-+1 ""-+,......,� [1 12[4 12 lo 1--+-+---+-+--bll>� [1 1112[2 1214[0

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f-f-l' 1-H++++t-H++-l++H++++-7 13 12 1--t--+--F�"- l++++t-t+t-tiHttt++t++t-7 12 13

J? H++++++-�+++++t++-7 11 13 lo 1--+--�i;>-[7 1+++++++-�++H++t+- 7 lo l•lo 1 14[2 !4

I--+-+-+-+-+-�-4T . 712 13[1

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Appendix 1, ponel7 of 10

o

Gamma roy <API>

Lithology Skidattic:

Core sr' � ' '!' 15C �� ··w P �

� 182� ·�

835

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1111111111 * c

hs4o 1-1-lH�I-1-1-lH�I<

Porosity CiO


do l. CCD SCI•

7128/6-1 col onhydrite

10

H�-t-t-+--J�pr t-t+tt+lrtttf ''d-i+tffirt 3 4 4 3 1 o o o o o � 440 1 1 1 0000

J t+++-tt+t-t+t 4420 1 2000 1

443 1 0 1 0000

� H-i-+++++++t lttt+l+tH 3 3 4 o 1 o o o o o H-t-t-++--ft"'l t+t++t++t-tt Hrtt-ttt++t

.,. tt+t+tt+ttf

l

4400 1 20000

44 1 002000 1

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l.J l!' l 11 l 11 l L4 4 0 1 1 2 1 0 0 0 rt-HH-t"'"'-� H-i+t-1-++ffi "'\l.Mffif-t++t 4 4 o 1 1 o o o o o

� H-i+t-1-++ffi

·� i t-+1-+++++++t

t--+-t-t-+--t-:tr[) t+tt+t+tttf �-r,_��H+H+H+H � t-+-+-t-+-��tt+t-t+tt-H-1 r+-r��1��+1 t--+--t--i-t--+--<1( t+tt+t+tttf t-+--t--t-t-+-:.·�� H+t+tt+ttf c

340 1 1 1 1 000 342000000 1 3440000000 34 1 1 1 0000 1 3400 1 1 0000 44 1 0 1 1 000 1 343000 1 000 442 1 00 1 003 440 1 1 1 000 1 344 1 00000 1 44 1 0 1 2 1 00 1 34 1 2 1 0 1 004 342 1 1 0 1 003 4422 1 1 0003 440 1 1 1 0003

1-HH-t-t-ti Hrtt++t++H 11rtttt1+tt 4 4 o 1 1 o o o o 1 4402 1 1 1 00 1

1-HH-t-t-tflffi-t+ffi-H tt-tti++t+tt 4 4 o 1 1 1 o o o o 1-HH-t-t-t_..ffi-ttffii-tfiii-HI-ttttlrtt 4 4 o 2 1 1 1 o o o 1-HH-t-t-+1,...-tt-ffi+tffir-HiffiH+ttlt+l 4 4 o 2 1 2 1 o o o

.--r-t-H,....-t-t--+�n-t-tt++t-tt-Htt-tt-tt-ttt-tt 4 4 o 2 1 2 1 o o o 1-1-H-t-1: 1-H+tffi+tt.+IH+ttlH+t 1 1 o o 3 3 2 o o 2

l� ]l

11 1 0 1 22 1 004 t-H+tffil-ttUftt+t-1-ttt 1 2 o 1 3 2 3 o o o H-if-t+ffiHi"itrH-t++t+t-t- 7 3 o o 3 1 2 o o o t-HI-ttt+ll-tt-tt-1-tt+t-t+t+ 7 o o o 3 4 1 4 o o


Appendix 1, panel8 of 10 Gamma ray

<A Pil



Appsnd]). 1, panel9 of 10 Gamma ray CAP l> Coro

15C tinl

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Finnmark carbonate p/atform, Barents Sea 223

Porosity (;/,)

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7128/6 1 -SCI+ �

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Gamma ray l�clitholagy l ��i Porosity <APil Core 1 ":' -,� Sl "" OD ,.,l�fl�' � 1 r 1 9 ' 135

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7128/4-1 CCD SCI • l§

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